Blackfin: support embedding the environment into loader files (LDRs)

For the most part, the Blackfin processor boots files in the LDR format
rather than binary/ELF files.  So we want to export the environment as a
raw blob to the LDR utility so it can embed it at the right location.

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