linux.bin, linux.bin.gz, and linux.bin.ub are phony targets to
generate a corresponding image under arch/microblaze/boot/.

simpleImage.% also works like a phony target, but a pattern that
contains '%' cannot be a phony target. I replaced it with equivalent
simpleImage.$(DTB).
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>