Check to see if `${CROSS_COMPILE}mkimage` exists and if not, fall back to
the standard `mkimage`

The Blackfin toolchain includes mkimage, but we dont want to namespace
collide with any of the user's system setup, so we prefix it with our
toolchain name.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Oleg Verych <olecom@flower.upol.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>