Allows building within the Arduino Source tree, and within the Arduino
IDE tree, as well as using CrossPack on Mac.
Adds README.TXT to track arduino-specific changes (and documents the
new build options.)
This addresses Arduino issue:
http://code.google.com/p/arduino/issues/detail?id=487
And optiboot issue
http://code.google.com/p/optiboot/issues/detail?id=1
(which can be thought of as a subset of the Arduno issue.)
Note that the binaries produced after these Makefile changes (using any
of the compile environments) are identical to those produced by the
crosspack-20100115 environment on a Mac.
(cherry picked from commit 2d2ed324b4)
