Simplify source finding and fix dependencies

Make the code finding sources to compile a bit simpler and fix the generation
of dependency files into the build directory, to avoid cluttering the src
directory.
Workaround gentoo's avr-size

Since the avr-size in cross-avr/binutils on Gentoo Linux doesn't have the
--format=avr or --mcu options (possibly related to the issue described here?
http://hackaday.com/2010/09/01/how-to-fix-avr-size-on-ubuntu-10-04/), add a
workaround to only supply those options if avr-size --help lists them.

Also, make the size target a .PHONY target and specify its prerequisites.