linux: use zImage by default on ARM

Since quite some time, the kernel and bootloader communities consider
zImage as the default format for kernel images on ARM, replacing
uImage. The load address information in uImage is no longer needed,
since the kernel is position-independent in terms of physical address,
except on a few old platforms. For most people, using zImage is simply
better/simpler, so let's switch to zImage as the default image format
on ARM.

All defconfigs are updated: 46 defconfigs no longer need to select
explicitly zImage because it's the default, and 16 defconfigs now need
to explicitly select uImage because that's no longer the default.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Matt Weber <matt@thewebers.ws>
Acked-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>