boot: Make initrd_prepare() semantically equivalent to combine_initrds()

boot: Make initrd_prepare() semantically equivalent to combine_initrds()

Currently, trying to boot images with type 1 entries generated by mkosi
with qemu freezes in the kernel EFI stub. I'm not going to pretend I
understand what's going on, but when I reported a similar problem with
UKIs, the fix was to rework the code in combine_initrds() in the stub
to behave like it does today. It seems that same fix was never applied
to systemd-boot's combine_initrds() function, so let's do that now to
fix the freezes I've been seeing trying to boot images with type 1 entries
in qemu.