postmarketos-mkinitfs: drop support for multiple kernel flavors

The part of postmarketos-mkinitfs supporting multiple kernel flavors was modeled after how it's done in Alpine. This part makes sense in Alpine, you can have multiple kernels installed at the same time (linux-edge, linux-lts, linux-virt, ...).

However, in postmarketOS we actually only install one kernel at a time for one device. Even for devices that support multiple kernels, we ask for the kernel to be installed in "pmbootstrap init" and then only install that one. Most of the android devices have a boot.img file, where we can just put one kernel in, too.

To really support multiple kernels, we would need a device-agnostic graphical boot menu, that loads the selected kernel with kexec. This does not seem feasible, and would likely increase the boot time.

The postmarketos-mkinitfs code would be more readable and maintainable, if we removed support for multiple kernel flavors. It would also make the usage easier for manually calling the script. We don't need to figure out the correct parameters anymore,

$ mkinitfs -o OUTFILE KERNELVERSION

becomes just:

$ mkinitfs

(parameters would be ignored for compatibility.)