Blackfin: convert specific pre/post config headers to common method

The Blackfin port was using asm/blackfin-config-{pre,post}.h to setup
common Blackfin board defines.  The common method now is to use config.h,
so convert blackfin-config-post.h to that.  Rename the still Blackfin
specific blackfin-config-pre.h to config-pre.h so the naming conventions
at least line up.

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