ipq40xx: arm: compressed: add appended DTB section

ipq40xx: arm: compressed: add appended DTB section

This adds a appended_dtb section to the ARM decompressor
linker script.

This allows using the existing ARM zImage appended DTB support for
appending a DTB to the raw ELF kernel.

Its size is set to 1MB max to match the zImage appended DTB size limit.

To use it to pass the DTB to the kernel, objcopy is used:

objcopy --set-section-flags=.appended_dtb=alloc,contents \
	--update-section=.appended_dtb=<target>.dtb vmlinux

This is based off the following patch:
https://github.com/openwrt/openwrt/commit/c063e27e02a9dcac0e7f5877fb154e58fa3e1a69

Signed-off-by: Robert Marko <robimarko@gmail.com>