If TARGET or EMBEDDED_TARGET == arm, set EMBEDDED variable,

If TARGET or EMBEDDED_TARGET == arm, set EMBEDDED variable,
which helps control some of the arm-specific bits a bit more
cleanly (but not really 'clean').

If BOARDNAME is defined (as is in the WANDBOARD configuration
RE uses), do some magic to work with the KERNCONF and BOARDNAME
to rename the file, making it a bit more intuitive for the
consumer to determine which they need.

Yes, it is ugly, that is why there is a big warning at the top.
It is, however, still much cleaner than the now 474-line shell
script, and this Makefile produces the hierarchy needed without
much evil.

MFC after:	1 week
X-MFC-Note:	needs all previous Makefile.mirror commits
Sponsored by:	The FreeBSD Foundation