archiso/mkarchiso: do not hardcode the kernel and initramfs file names

All kernels and initramfs images from airootfs are copied to ${install_dir}/boot/ on both ISO 9660 and FAT.
This allows providing multiple kernels. The kernel package needs to be added to packages.x86_64 and the boot loader configuration files created/adjusted appropriately.

Support all possible microcode initrd file names.