use minigzip instead of system gzip in the build

use minigzip instead of system gzip in the build

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.