microblaze: adjust the help to the real behavior

"make ARCH=microblaze help" mentions simpleImage.<dt>.unstrip,
but it is not a real Make target. It does not work because Makefile
assumes "system.unstrip" is the name of DT.

$ make ARCH=microblaze CROSS_COMPILE=microblaze-linux- simpleImage.system.unstrip
  [ snip ]
make[1]: *** No rule to make target 'arch/microblaze/boot/dts/system.unstrip.dtb', needed by 'arch/microblaze/boot/dts/system.dtb'.  Stop.
make: *** [Makefile;1060: arch/microblaze/boot/dts] Error 2
make: *** Waiting for unfinished jobs....

simpleImage.<dt> works like a phony target that generates multiple
images. Reflect the real behavior. I removed the DT directory path
information because it is already explained a few lines below.

While I am here, I deleted the redundant *_defconfig explanation.

The top-level Makefile caters to list available defconfig files:

  mmu_defconfig            - Build for mmu
  nommu_defconfig          - Build for nommu

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