Blackfin: add init.elf helper code

This creates a standalone ELF that executes just the Blackfin initcode.
This is useful for people who want to program the low level aspects of
the CPU (memory/clocks/etc...) and can easily be used with JTAG for
quick booting while developing.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>