Allow MINIGZIP override for default ramdisk compression tool

The Android build system only supports gzip for ramdisks, despite
the kernel itself supporting other options. Allow per-device
configs to specify their own tool, like so:

MINIGZIP := $(shell which lzma)
MINIGZIP := $(shell which bzip2)
...etc.

Change-Id: I0bc754bd2a73bfb4166228b420c8f013808c82f2