kbuild: remove UIMAGE_IN and UIMAGE_OUT

The only/last user of UIMAGE_IN/OUT was removed by commit 4722a3e6b716
("microblaze: fix multiple bugs in arch/microblaze/boot/Makefile").

The input and output should always be $< and $@.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>