main/postmarketos-mkinitfs: provide initramfs-generator

Alpine provides a 'initramfs-generator' thing now that the alpine linux-* depend on explicitly. This sets postmarketos-mkinitfs to be the preferred provider of that virtual package.

Fixes:

    ERROR: unable to select packages:
      postmarketos-mkinitfs-1.4.1-r5:
        conflicts: mkinitfs-3.6.1-r2[mkinitfs=3.6.0]
        satisfies: postmarketos-base-20-r1[postmarketos-mkinitfs]
                   postmarketos-base-nofde-20-r1[postmarketos-mkinitfs]
                   .pmbootstrap-20220610.215815[postmarketos-mkinitfs]
                   .pmbootstrap-20220610.215815[mkinitfs]
      mkinitfs-3.6.1-r2:
        conflicts: postmarketos-mkinitfs-1.4.1-r5
        satisfies: .pmbootstrap-20220610.215815[mkinitfs]
                   linux-rpi-5.15.45-r1[initramfs-generator]
      .pmbootstrap-20220610.215815:
        masked in: cache
        satisfies: world[.pmbootstrap=20220610.215815]

Also see:

    30d7b88324 (aports) main/mkinitfs: provide initramfs-generator

    4fac61c826 (aports) main/linux-lts: replace mkinitfs with initramfs-generator