postmarketos-initramfs: init_functions.sh should handle rootfsopts

Describe your issue

What's the expected behaviour?

Adding a boot parameter like rootfsopts=subvol=@,noatime to the kernel command line should be passed through and used by mount_root_partition() to allow the initial root mount using custom options (in this case, specifying which btrfs subvolume to mount).

Additionally, it would be nice if there was a better error when a filesystem was mounted but the expected entries on it don't exist:

Mount root partition (/dev/sda1) to /sysroot (read-only)
Detected btrfs filesystem
[   10.264708] BTRFS: device label pmOS_root devid 1 transid 15 /dev/sda1 scanned by mount (565)
[   10.272929] BTRFS info (device sda1): using crc32c (crc32c-generic) checksum algorithm
[   10.279787] BTRFS info (device sda1): using free space tree
ERROR: unable to mount root partition!

The mount succeeded, but because the wrong subvolume was mounted the only directories visible are @ and @subvolumes in my example.

What's the current behaviour?

There's no way to pass options to the mount of the root filesystem, therefore no way to select a btrfs subvolume to mount without setting it as the default subvolume.

How to reproduce your issue?

Try to boot a device with a btrfs root where the root filesystem is not installed into the root subvolume.

What device are you using?

odroid-xu4

On what postmarketOS version did you encounter the issue?

• edge (master branch)

• v23.06

• v22.12 (Supported until 2023-07-06)

• I confirm that the issue still is present after running sudo apk upgrade -a

How did you get postmarketOS image?

• from https://images.postmarketos.org
• I built it using pmbootstrap
• It was preinstalled on my device

What's the build date of the image? (in yyyy-mm-dd format)

2023-07-29