This allows TARGET_ARCH_VARIANT to be set by the vendor before we choose the
architecture in core/combo/select.mk.
Also add a primitive armv7-a.mk for turning on hardware floating point.
Use zlib's minigzip utility, built as part of our source tree, instead of
whatever installation of GNU gzip happens to be on the user's machine.
Using zlib's deflater, which is nicely available as a library (unlike
GNU gzip's deflater) will ultimately let us do binary patches to the
boot and recovery images.
