[uboot4extlinux] do not use fallback images as initrd
Linux-libre kernels for ARM do not have fallback images (although I can add them, but that's another discussion). And, as they are not present, uboot4extlinux fails to boot using the default configuration (happened to me with uboot4extlinux-bananpi in my BPi M1).
Plus, they have non-existing kernels, which were removed a long time ago. Please, before updating the extlinux.conf make sure about what you put there because, e.g., linux-libre-hardened is only for x86_64.
Maybe instead of removing the fallback entries I could add them in the armv7 mkinitcpio presets and the uboot maintainer would just have to add the non-fallback entries in extlinux.conf (or without initrd entries like bedore, in fact, idk why they were added)