build: image: add append-dtb-elf helper

build: image: add append-dtb-elf helper

The mips kernel vmlinux image supports adding an empty ELF section
for DTB to be later inserted into with MIPS_ELF_APPENDED_DTB.

This ELF + inserted DTB image can then be directly booted on some
devices.

Example usage:
image/subtarget.mk:
KERNEL_NAME := vmlinux.elf
KERNEL_INITRAMFS_NAME := vmlinux-initramfs.elf
KERNEL := kernel-bin | append-dtb-elf

On mt7621 memory size needed to be manually specified.

Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>