vc4: Use a wrapper file to set VC4_BUILD_NEON instead of CFLAGS.

vc4: Use a wrapper file to set VC4_BUILD_NEON instead of CFLAGS.

Android.mk was setting the flag across the entire driver, so we didn't
have non-NEON versions getting built.  This was going to be a problem with
the next commit, when I start auto-detecting NEON support and use the
non-NEON version when appropriate.

Reviewed-by: Rob Herring <robh@kernel.org>