build: consolidate fake uImage header build commands

build: consolidate fake uImage header build commands

Merge the two existing functions and use a parameter for the type
header field.

It updates the syntax of the former mpc85xx fake ramdisk header
command to be compatible with mkimage from u-boot 2018.03 and fixes the
build error spotted by the build bot.

Signed-off-by: Mathias Kresin <dev@kresin.me>