default-distrovars: set default KERNEL_IMAGETYPE(S)

With the change from assuming kernels will be named "vmlinuz"
everywhere, to instead using KERNEL_IMAGETYPE, we require that
KERNEL_IMAGETYPE is set to something. Instead of setting the default in
multiple individual files, set it in default-distrovars.inc.

x86(-64) arches get bzImage as the default. Others get zImage as per
meta/conf/documentation.conf.

Also set KERNEL_IMAGETYPES, as we will eventually be switching away from
KERNEL_IMAGETYPE.

Thanks to Andre McCurdy <armccurdy@gmail.com> for the arch defaults
part.

(From OE-Core rev: a57d784211a39587538094425ee0246e9ddfbf9d)

Signed-off-by: California Sullivan <california.l.sullivan@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/syslinux.bbclass

@@ -21,7 +21,6 @@ do_bootimg[depends] += "${MLPREFIX}syslinux:do_populate_sysroot \
                         syslinux-native:do_populate_sysroot"
 
 ISOLINUXDIR ?= "/isolinux"
-KERNEL_IMAGETYPE ??= "bzImage"
 SYSLINUXDIR = "/"
 # The kernel has an internal default console, which you can override with
 # a console=...some_tty...