main/postmarketos-initramfs: always resize rootfs during boot

At the moment, the root file system is only resized to span the entire partition if we previously also resized the partition itself. However, if a rootfs image without subpartitions is flashed to a pre-existing partition this means that the installation only has very limited space available without manual resizing.

Such a setup is used for devices using the "fastboot-bootpart" flasher (e.g. asus-me176c), where boot and root images are flashed to two separate partitions and the device boots directly from the boot partition.

Avoid this problem on these devices by always resizing the root file system on each boot. All the resize tools exit early if the file system is already expanded to the maximum size. It's faster to call resize2fs unconditionally instead of manually checking if the file system needs resizing before. Similar tooling like systemd-growfs also resizes unconditionally.

The other two commits cleanup/optimize the resizing implementation a bit (see commit log).