microblaze: Makefile cleanup

Reviewed the Makefile on request by Michal and this is the resulting changes.

o Use ':=' for assignmnet so we do not re-evaluate for each use
o Use $(shell echo xxx) to remove ""
o Replaced CFLAGS_KERNEL with KBUILD_CFLAGS
  The settings are equally relevant for modules and the linked kernel
o Dropped LDFLAGS_BLOB - it is no longer used
o Refactored assignmnets to libs-y and core-y
o Use MMU for the MMU specific extension. "MMUEXT" was hurting my eyes
  and I did not wanted it spread to m68k

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Simek <monstr@monstr.eu>