CPU_MODELS parameter changes.
arch/SConscript:
Fix typo in comment.
cpu/SConscript:
Convert exec signature generator to Action so we can add dependency
on CPU_MODELS environment var.
Print nicer string while we're at it.
Also add some comments.
--HG--
extra : convert_revision : bcb38a7941943cf071dac34cdbb2ece5456b8620