kbuild: Rename UIMAGE to MKIMAGE

U-Boot uses the 'mkimage' tool to produce various image types,
not only uImage image type. Rename the invocation name from
UIMAGE to MKIMAGE.

The following command was used to do the replacement:
git grep 'quiet_cmd_mkimage.* = UIMAGE' | cut -d : -f 1 | \
 xargs -i sed -i "s@\(quiet_cmd_mkimage\)\(.*\) = UIMAGE @\1\2 = MKIMAGE@" {}

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>