build: Introduce concept of SoC specific build flags

Introduce a soc-cflags, soc-cxxflags, and soc-aflags as a means for
SoC specific compiler flags to be set without manipulating Kbuild
options directly.

Change-Id: I2c8f5019fb237429e59717ef96bd4251a61dc1a5
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>