kernel-uboot.bbclass: Use vmlinux.initramfs when INITRAMFS_IMAGE_BUNDLE set

kernel-uboot.bbclass: Use vmlinux.initramfs when INITRAMFS_IMAGE_BUNDLE set

vmlinux file doesnot have the initramfs image when INITRAMFS_IMAGE_BUNDLE was set.
Use vmlinux.initramfs in uboot_prep_kimage when INITRAMFS_IMAGE_BUNDLE set
based on the implementation in kernel.bbclass do_bundle_initramfs function,
https://github.com/openembedded/openembedded-core/blob/master/meta/classes/kernel.bbclass#L316-L317
to be able to use proper linux.bin file in creation of fitImage.

Signed-off-by: Raju Kumar Pothuraju <raju.kumar-pothuraju@xilinx.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>