kernel: fix CopyImage function with Per Device Rootfs

kernel: fix CopyImage function with Per Device Rootfs

Some target define custom kernel images with KERNEL_INITRAMFS_NAME to
reference .elf variant of vmlinux.

With Per Device Rootfs, the expected format is
image.suffix.extension.ROOTFS_ID, while in CopyImage we are currently
generating images with image.suffix.ROOTFS_ID.extension making some
target failing.

Fix CopyImage function to correctly follow the expected pattern.

Fixes: 97fd059e7e6a ("image: respect TARGET_PER_DEVICE_ROOTFS for initramfs")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>