Blackfin: propagate target cpu defines when building embedded env

Since we're no longer extracting the env from the target ELF file (since
upstream wouldn't take that change), we're back to the problem of cpu
defines not properly propagating to the env setup stage.  So the embedded
env built by the host compiler doesn't match the one that is linked into
the u-boot env.

Reported-by: Vivi Li <vivi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>