We don't need Image.gz - we only need Image, which we post-process by
compressing using lz4 and then package the result of that.

This shaves about 250ms off each build.

While we could specify Image.lz4 here, I've opted not to do that, as
Kbuild uses compression level -9 whereas we use --fast, which is about
1.2 seconds faster in my environment. The size difference between the
two is less than 6MiB, which results in a barely measurable difference
in time when uploading to the device using fastboot boot.

Signed-off-by: André Draszik <andre.draszik@linaro.org>