Add a "remove" command to cbfstool

Add a "remove" command to cbfstool

This command removes the first file it finds with the given name by changing
its type to CBFS_COMPONENT_NULL and setting the first character of its name to
a null terminator. If the "files" immediately before or after the target file
are already marked as empty, they're all merged together into one large file.

Change-Id: Idc6b2a4c355c3f039c2ccae81866e3ed6035539b
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Ronald G. Minnich <rminnich@google.com>
Reviewed-on: http://review.coreboot.org/814
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>