[PATCH] kbuild/ppc: tell when uimage was not built

Tom Rini said:
  Note that there is still a trivial'ish change to make.  When mkimage
  doesn't exist on the host we should say "uImage not made" or
  something similar.

So I did like Tom asked.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>