Blackfin: fix bd_t handling

The recent global data changes (making the size autogenerated) broke the
board info handling on Blackfin ports as we were lying and lumping the
bd_t size in with the gd_t size.  So use the new dedicated bd_t size to
setup its own address in memory.

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