Guards against errors that are hard to track down:

Guards against errors that are hard to track down:
- if crt0s is empty (eg. because crt0-y is still used),
  break the build, and say where that behaviour changed
- if a stage is unusable for cbfstool because it's placed
  outside the ROM space (linked to 0 is somewhat notorious),
  warn about it, give some hints and exit instead of crashing.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Stefan Reinauer <stefan.reinauer@coresystems.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5077 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1