armv7-a build: allow for board customization of CPU/FPU target

to cover the entire gamut of CPU and FPU compilation types possible
with ARMv7 (uniprocessor vs SMP, no-FPU vs VFPv3 vs Neon, A8 work-
arounds required vs unnecessary), the compilation codepath needs to
be specified by the BoardConfig.mk to ensure that the OS matches the
target hardware.

add TARGET_ARCH_VARIANT_FPU, TARGET_ARCH_VARIANT_CPU and
TARGET_CPU_SMP makefile variables to allow for target-directed
compilation flags.

Change-Id: I46cd5dbe3e3c5e881473b2e97fd4cee7cd5c29a8