[coreboot] cbfstool warnings

[Myles Watson]

Since cbfstool is so simple right now, it makes sense to have -Werr in
CFLAGS so that we are more careful with the code.  It will be a bigger
pain later.

I didn't want to touch lzma so the tools directory has its own CFLAGS now.

I chose to make all sizes int.  They could have been unsigned int.  If
anyone really cares we can change it.  It just needs to be consistent.

I cleaned up some of the parameters (usage and checking.)  There's no
need to have an empty add call, etc.

Signed-off-by: Myles Watson

Thanks,
Myles