Draft: temp/u-boot-pinephone: drop u-boot in favor of Tow-Boot

We've moved the PPP to Tow-Boot, let's do the same with the OG PP! This allows us to drop our custom u-boot package and boot with both extlinux and UEFI, and it standardizes booting over both the PinePhone's.

Eventually I'd like us to move to UEFI booting but we have to figure out how to do that properly without breaking existing installations, thus we'll stay on extlinux for now.

UEFI support is added as a separate commit, not sure if it will break existing installations.

Blockers that have to be figured out before this get merged as to not break some custom setups:

• #1531: load pmOS_root / pmOS_boot partitions by UUID instead of label (-> also fixes #1485)
• !2665 (merged)
  • Does Tow-Boot work as is with the Vccq hardware mod? I can't test this myself
• !1998 (merged)
  • Can we still provide the option to choose the RAM hz when using Tow-Boot? Again, I can't test this myself
  • We'll just drop this feature and use 528 MHz, there was not enough gain from this, see comment below.
• should make everything work with on-device installer: make sure everything works with installing to SD, installing to eMMC, adjust if needed. (after #1531)