ipq40xx: wpj428: switch to zimage to fit kernel partition

ipq40xx: wpj428: switch to zimage to fit kernel partition

Like with some other ipq40xx devices, the kernel image size for the WPJ428
is limited in stock u-boot. For that reason, the current release doesn't
include an image for the board.
By switching to the zImage format, the kernel image size is reduced which
re-enables the build process. The image boots and behaved normally through
a few days of testing.

Before the switch to kernel version 6.1, it was possible to reduce the
image size by enough when disabling UBIFS and its otherwise unneeded
dependencies.

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>