ANDROID: ARM: Use zImage instead of Image

Nothing seems to support booting uncompressed ARM 32-bit kernels, and
everything supports zImage, so change the default on 32-bit to zImage.

More information about this:
http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html



Bug: 165410044
Test: booted zImage on cuttlefish
Change-Id: I130d38737643dc092900e57e117ac268f6a9bd07
Signed-off-by: Alistair Delva <adelva@google.com>