Another indirection for normal/fallback bootblock

Another indirection for normal/fallback bootblock

Provide a way to redefine the names of normal and fallback via CBFS.
This way updates can use some more expressive naming scheme (numbers,
dates, version numbers) and replace the coreboot-stages file to
point to the new version (with the current version as new "old").

If coreboot-stages doesn't exist, the default behaviour remains to
use "normal" and "fallback".

Change-Id: I77c134d79ed95831ad5098b7663c15e95d3b5a2a
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/589
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>