[coreboot] CBFS build directory

[Myles Watson]

The object files for cbfs clutter target directories.  This fixes it.  I
also added a cbfstool-clean target to the Makefile.

before:

myles@orangutan:targets$ ls emulation/qemu-x86/qemu-x86
Makefile           cbfstool      coreboot.rom.bootblock  fs.o
resize.o
Makefile.settings  cbfstool.o    create.o                normal       romcc
add.o              config.py     delete.o                payload.elf  tools
bootblock.o        coreboot.rom  extract.o               print.o      util.o

after:
myles@orangutan:targets$ ls emulation/qemu-x86/qemu-x86
Makefile           cbfs       coreboot.rom            normal       romcc
Makefile.settings  config.py  coreboot.rom.bootblock  payload.elf

I'm not particular about the name.  I chose cbfs because it is short.
cbfs-tools/ or something else would have been fine too.

Signed-off-by: Myles Watson

Thanks,
Myles