dda_queue: optimize enqueue().

Suggestion by ItsDrone, see
http://forums.reprap.org/read.php?146,174364,174364#msg-174364
Wrap EEPROM storage related stuff in #ifdef EECONFIG.

Saves a whopping 600 bytes. Let's cross fingers stuff still works.
It should, using the hardcoded default values.
Rewrite upper half of the Makefile.

We have now Makefile-AVR - AVR specific stuff - and
Makefile-common - common build instructions.

This effort is the begin of preparing Teacup for ARM targets.
To build the target, copy or link Makefile-AVR or Makefile-ARM
(depending on your target) to Makefile.
README: shorten drastically to avoid documentation duplication.
Makefiles: build into a dedicated directory.

This makes finding source files for editing quite a bit simpler.
config templates: bring in EECONFIG.

Recently EEPROM storage was wrapped in #ifdefs, so, to get it back
by default, we have to enable it.
Makefiles can have various names now.