ar71xx: mikrotik: cleanup nand image build code

ar71xx: mikrotik: cleanup nand image build code

Use the LOADER_TYPE variable to specify that we need the elf preloader
and append the loader via the corresponding build recipe. It allows to
enable initramfs images again for mikrotik NAND images, which caused a
build error before.

Add the minor header only to the kernel of the sysupgrade images, as it
is only required for the bootloader to find the kernel on flash.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 5260c1003cee20b004f4628c72ef3985885ca901)