Add -DJZ_VERSION to the CPPFLAGS

Also cleaned up the Makefile.
Use a common head.S file instead of one for each config
Set the stack address to the end of the data cache

When booting up, the JZ loads the first 8 KiB of the boot device
to the instruction cache and the data cache; which means that
everything up to 0x2000 is already in cache. It makes sense to
use 0x2000 as the stack address.