mkarchiso: skip including external microcode images if the initramfs file contains early_cpio
The early uncompressed CPIO archive containing microcode update files
can be part of the initramfs file. To avoid wasting space, first check
if the initramfs file contains early_cpio
and only copy external
microcode initramfs images if it does not.
This does not change airootfs/etc/mkinitcpio.conf.d/archiso.conf
and boot loader configuration. Will still use microcode initramfs images as external files. This will most likely change in the future.