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>
diff --git a/arch/microblaze/kernel/Makefile b/arch/microblaze/kernel/Makefile
index f4a5e19..d487729 100644
--- a/arch/microblaze/kernel/Makefile
+++ b/arch/microblaze/kernel/Makefile
@@ -17,4 +17,4 @@
 obj-$(CONFIG_MODULES)		+= microblaze_ksyms.o module.o
 obj-$(CONFIG_MMU)		+= misc.o
 
-obj-y	+= entry$(MMUEXT).o
+obj-y	+= entry$(MMU).o