postmarketos-mkinitfs: fix unnecessary runs in "pmbootstrap install", use cross-native2

• Adjust the trigger script, so mkinitfs runs only once during pmbootstrap install (when it gets explicitly called at the end), giving it a nice speed up.
• Use cross-native2 for cross compilation to make it faster (see https://docs.postmarketos.org/pmbootstrap/cross_compiling.html).
• Fix a "cleaning up old boot files" print that currently gets printed even if not files get deleted.

See commit messages for details.

CC: @craftyguy, @caleb

Depends on: pmbootstrap!2565 (merged)