Update scripts/Makefile.fwinst to cope with older make

Also fix unwanted rebuilds of the firmware/ihex2fw tool by including
the .ihex2fw.cmd file when present.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reported-and-tested-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>