firmware/Makefile: force recompilation if makefile changes

firmware/Makefile: force recompilation if makefile changes

If you modify the target asm we currently do not force the recompilation
of the firmware files.  The target asm is in the firmware/Makefile, peg
this file as a dependency to require re-compilation of firmware targets
when the asm changes.

Link: http://lkml.kernel.org/r/20170123150727.4883-1-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <mmarek@suse.com>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tom Gundersen <teg@jklm.no>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>