cbfs/payload: fix reading payload body
cbfs/payload: fix reading payload body

We got the order wrong; rename variables to what they are in context,
print more info for debugging, and tolerate an empty payload body.

Signed-off-by: Daniel Maslowski <59bd0a3ff43b32849b319e645d4798d8a5d1e889@orangecms.org>

linuxboot/fiano