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