disable caches before booting an app for Blackfin apps

It isn't generally save to execute applications outside of U-Boot with caches
enabled due to the way the Blackfin processor handles caches (requires
software assistance).  This patch disables caches before booting an ELF or
just booting raw code.  The previous discussion on the patch was that we
wanted to use weaks instead, but that proved to not be feasible when multiple
symbols are involved, which puts us back at the ifdef solution.  I've
minimized the ugliness by moving the setup step outside of the main function.

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