Fix 'make clean' and .gitignore for firmware/ directory.

Provide a .gitignore file, and fix a typo which prevented some of the
generated binary files from being removed on 'make clean'.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reported-and-tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>