libmnflash: add README, man page

Building the man page requires pod2man from Perl, configure checks
for that and the manage is not installed when pod2man is missing.
bootloader: shorten target names (tiny-X -> tX)
bootloader: Add install target to the Makefile

"make install" installs the built hex and elf files on the system - by
default to "/usr/local/share/micronucleus/<target name>". The idea is
that the upload utility then can then find the right variant for a
given hardware.

The installation path can be influenced by passing "prefix" and "datarootdir"
as arguments to "make". "DESTDIR" is supported too.
bootloader: move MICRONUCLEUS_WIRING to the makefile includes.

This makes it easier to define a new target that has the same vUSB wiring
but different other features (jumper, led ...)
bootloader: assign wiring number to the jumper modes

Using a jumper to control the bootloader behavior is most certainly a different
wiring - one needs a specialized hardware to use this.

Not sure how to deal with the four software related options.
Application data pointer in mnflash_usb_t.

There was an left-over field "i2c_addr" in mnflash_usb_t, which came from
the application the usb stuff has been written for and which served the
same purpose there. Removed that.
Re-enable accidently disabled firmware upload in mnflash.
Fix chip-id for ATtiny 1634.
Fix missing @exec_prefix@ in Makefile.in

... caused libs/binaries to end up in /lib and /bin.
Get rid of trailing ne lines in messages.

Handle that in the default implemenataions for mnflash_msg() and
mnflash_error() instead. This makes it a bit easier to actually overwrite
those functions (slog for example).
Message/error output can nowbe disabled.
Add debug logging facility

For completeness and because I need that elsewhere, it is not (yet) used by the
lib.

Debug logging is disabled by default but can be enabled through
"mnflash_set_debug_log()", which can be used to set a function that will do the
actual logging or by using "mnflash_enable_debug(1)", which makes the debug
output to use the same function as used for normal messages.
Note device disconnect.

When libusb reports an ENODEV, mark that in the device handle struct. The
application should then attempt to reconnect the device.