microblaze: fix multiple bugs in arch/microblaze/boot/Makefile

This commit fixes some build issues.

The first issue is the breakage of linux.bin.ub target since commit
ece97f3a ("microblaze: Fix simpleImage format generation")
because the addition of UIMAGE_{IN,OUT} affected it.

make ARCH=microblaze CROSS_COMPILE=microblaze-linux- linux.bin.ub
  [ snip ]
  OBJCOPY arch/microblaze/boot/linux.bin
  UIMAGE  arch/microblaze/boot/linux.bin.ub.ub
/usr/bin/mkimage: Can't open arch/microblaze/boot/linux.bin.ub: No such file or directory
make[1]: *** [arch/microblaze/boot/Makefile;14: arch/microblaze/boot/linux.bin.ub] Error 1
make: *** [arch/microblaze/Makefile;83: linux.bin.ub] Error 2

The second issue is the use of the "if_changed" multiple times for
the same target.

As commit 92a47286

 ("x86/boot: Fix if_changed build flip/flop bug")
pointed out, this never works properly. Moreover, generating multiple
images as a side-effect is confusing.

Let's split the build recipe for each image.

simpleImage.<dt>*.unstrip is just a copy of vmlinux.

simpleImage.<dt> and simpleImage.<dt>.ub are created in the same way
as linux.bin and linux.bin.ub, respectively.

I kept simpleImage.* recipes independent of linux.bin.* ones to not
change the behavior.

Lastly, this commit fixes "make ARCH=microblaze clean". Previously,
it only cleaned up the unstrip image. Now, all the simpleImage files
are cleaned.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>