build: allow adding kernel specific cflags

use TARGET_EXTRA_CFLAGS if available but allow using separate KERNEL_CFLAGS
this is for mako so i can tune the gcc4.7 built kernel for cortex-a15
while tuning the gcc4.6 built os for cortex-a9

Change-Id: I3a3410a99d19161254c297d8060ccee97ec51a68
envsetup: dont include device vendorsetup files

Change-Id: I8b77be704b42a1d4d3edb7472ac4ad95a4c58a0c