Makefile-AVR: Update MCU_TARGET=at90usb1286 conditional.
Give users a hint in case they obviously forgot to read instructions.
heater.c: Limit PID I term with conditional integration.
heater.c: Enable more anti-windup with PID_CONDITIONAL_INTEGRATION.
heater.c, config.default.h: Make PID_CONDITIONAL_INTEGRATION non-optional.

See
https://github.com/Traumflug/Teacup_Firmware/issues/74#issuecomment-38999466
Get rid of E_STARTSTOP_STEPS.

This meant to be a firmware-provided retract feature but was
never really supported by G-code generators. Without their support
(by issueing M101/M103), it's pretty hard to detect extrusion
pauses, so this feature simply has no future.

As this was on by default, it saves over 200 bytes binary size
in a default configuration.
Implement adaptive homing feedrates.

For now for X min only, but it works excellently already.
Tested quite a few combinations and raising acceleration
or endstop clearance raises homing feedrate just as expected.

Quite a chunk of the code is for testing the given configuration,
only. A thing which would ideally be done for every macro
used in each code file.
config.*.h: distribute adaptive homing feedrate to all templates.
home.c: adaptive homing feedrates for all axes.
home.c: remove some redundant preprocessor stuff.

Binary size is exactly the same, to the optimizer apparently
manages to drop empty functions.
gcode_process.c: more preprocessor conditions for homing movements.

Well, optimizer isn't _that_ smart. It apparently removes
empty functions in the same compilation unit ( = source code file),
but not ones across units.

This saves 10 bytes binary size per endstop not used, so 30 bytes
in a standard configuration. All without any drawbacks.
DDA: Convert TARGET axis vars to array.

In preparation for more efficient and scalable code using axis-loops
for common operations, add two new array-types for signed and unsigned
32-bit values per axis. Make the TARGET type use this array instead of
its current X, Y, Z, and E variables.

Traumflug notes:

- Did the usual conversion to spaces for changed lines.

- Added X = 0 to the enum. Just for peace of mind.

- Excellent patch!

Initially I wanted to make the new array an anonymous union with the
old variables to allow accessing values both ways. This way it would
have been possible to do the transition in smaller pieces. But as
the patch worked so flawlessly and binary size is precisely the
same, I abandoned this idea. Maybe it's a good idea in other areas.
DDA: Convert more axis variables to arrays.

Many places in the code use individual variables for int/uint values
for X, Y, Z, and E.  A tip from a comment suggests making these into
arrays for scalability in the future. Replace the discrete variables
with arrays so the code can be simplified in the future.
home.c: don't move to zero after homing to max endstop.

This can be counterproductive if the actual zero point is
outside the available build room. For example, if an additional
bed probing is going to happen. It also costs quite some
time on the Z axis. If you actually  want this behaviour,
send a simple G0 XYZ after homing.
home.c: no need to turn off Z axis here.

This is done in dda.c already, see dda.c, line 678.
DDA: convert um_to_steps_* to generic implementation.

A generic implementation here will allow callers to pass the
target axis in as a parameter so the callers can also be made more
generic.

Traumflug notes:

Split out application of the new implementation in dda.c into its
own commit.

This actually costs 128 bytes, but as we can access axes from within
a loop now, I expect to get more savings elsewhere.

Interestingly, binary size is raised by another 18 bytes if

  um_to_steps(int32_t, enum axis_e)

is changed to

  um_to_steps(enum axis_e, int32_t)

even on the 8-bit ATmega. While putting the axis number to the
front might be a bit more logical (think of additional parameters,
the axis number position would move), NXP application note
AN10963 states on page 10ff, 16-bit data should be 16-bit aligned
and 32-bit data should be 32-bit aligned for best performance.
Well, so let's do it this way.
DDA: use new generic um_to_steps_* in dda_new_startpoint().

This was contributed by Phil Hord as part of another commit.

It saves 168 bytes, to it more than outweights the overhead of
introducing a generic implementation already.
DDA: Move axis calculations into loops, part 1.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Traumflug notes:

Split this once huge commit into smaller ones for ease of
reviewing and bisecting (in case something went wrong).

Part 1 is to put dda_create() distance calculations into loops.
This reduces binary size by another whopping 756 bytes.
dda_maths.h: remove now obsolete um_to_steps_[xyze].
DDA: Move axis calculations into loops, part 2.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.

Part 2 is moving maximum speed limit calculations into loops.
Binary size another 160 bytes smaller.