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>

include/image-commands.mk

@@ -142,6 +142,13 @@ define Build/append-dtb
 	cat $(KDIR)/image-$(firstword $(DEVICE_DTS)).dtb >> $@
 endef
 
+define Build/append-dtb-elf
+	$(TARGET_CROSS)objcopy \
+		--set-section-flags=.appended_dtb=alloc,contents \
+		--update-section \
+		.appended_dtb=$(KDIR)/image-$(firstword $(DEVICE_DTS)).dtb $@
+endef
+
 define Build/install-dtb
 	$(call locked, \
 		$(foreach dts,$(DEVICE_DTS), \