Add a UBOOT_MKIMAGE and UBOOT_MKIMAGE_SIGN variables to allow specifying
an alternative uboot-mkimage executable (or wrapper script/function).
(From OE-Core rev: aee5bac02eff28a75fa1eee646bc511984013aa4)
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>