main/postmarketos-initramfs: simplify and improve the boot process

Depends: !3230 (closed)

Rework the initramfs to use mdev, exposing /dev/disk/by-partlabel symlinks, as well as simplify and optimise the partition searching. Lastly, ensure that when device-mapper is used, devices are always mounted from /dev/mapper rather than /dev/dm-* and that device-mapper uses the /dev/disk/by-* symlinks. This ensures that well-known names are used everywhere rather than arbitrary and semi-random block device numbers.

This MR also fixes #2235 (closed), the recent issue where the root partition wasn't being resized on some devices.

TODO

Before merging this, it needs to be tested on a few different platforms, I have only validated it on SDM845 with subpartitions.

• Older Qualcomm (lk2nd)
• Pine64 devices
• Chromebook(s)
• EFI platforms
• X86
• Exynos 4