configs: zynq: revive BR2_LINUX_KERNEL_UIMAGE

Commit 5c67cb1d ("linux: use zImage by default on ARM") changed
the default kernel image, but missed to update Zynq defconfigs.

U-Boot on Zynq boards still loads uImage, so BR2_LINUX_KERNEL_UIMAGE
should be defined to generate uImage.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>