postmarketos-base: depend on postmarketos-initramfs (MR 3895)

Changes the dependency from pmos-mkinitfs to pmos-initramfs, so now the
dependency chain for boot config packages is:

    base -> postmarketos-initramfs -> postmarketos-mkinitfs -> boot-deploy

Sorting dependencies this way makes sense because "base" just needs an
initramfs, it doesn't care about the implementation (that currently uses
"postmarketos-mkinitfs.")

[ci:ignore-count]
[ci:skip-build] already successfully built in CI