support rootfs-as-initramfs
Implement support for stuffing the entire rootfs into an initramfs archive. This is useful in some cases for testing.
running "pmbootstrap install --initramfs" will output a regular postmarketOS boot partition, except the initramfs IS the root partition.
A cut down "none" UI install produces a ~170mb file with firmware and systemd (using a patched trailblazer package to not pull in unnecessary firmware).
Additionally, automatically install and enable the new postmarketos-network-gadget package so that the USB network gadget will be set up during boot as normal.