rpi-resizerootfs: switch the root filesystem resizing away from a systemd oneshot service

Switch away from using a systemd service for the initial root resize.
Instead, we resize the root partition and filesystem in the initrd.

To simplify things, the initrd script will check whether it should resize
the partition on every boot. It does this by checking if the entire disk
(ignoring an empty 4MB) is in use.  However, the scripts themselves are
deleted from the system after the initrd is generated. After the image
is installed, the resize script should exist only in the initrd. When the
kernel gets upgraded (eg, for a security update) or a new initrd is generated
due to a package install, the new initrd will not contain the resize script.
At that point, nothing will remain from the image's initial resize
bootstrapping process.

This process (but not the scripts) is similar to what cloud-initramfs-growroot
does. However, that particular package has an indirect dependency on Python,
and we don't necessarily want that overhead in our images just for resizing.

rootfs/etc/systemd/system/rpi-resizerootfs.service

@@ -1,13 +0,0 @@
-[Unit]
-Description=resize root file system
-Before=local-fs-pre.target
-DefaultDependencies=no
-
-[Service]
-Type=oneshot
-TimeoutSec=infinity
-ExecStart=/usr/sbin/rpi-resizerootfs
-ExecStart=/bin/systemctl --no-reload disable %n
-
-[Install]
-RequiredBy=local-fs-pre.target