Towboot PinePhone compatibility

[ArenM]

The u-boot boot script thinks it's booting from emmc even if it's booting an sd card when using tow-boot. It seems to be possible to resolve this by replacing mmc_bootdev with devnum.

This also makes it possible to remove the uboot-pinephone package when using tow-boot.

I'm suspicious of the code duplication this causes, and wonder if it would be better to use something like Debian's u-boot-menu script, Unfortunately that doesn't support device tree overlays yet, so that might be a project for a later time.

[ArenM]

Marking as ready since #398 got fixed

[ArenM]

I've tested this to make sure it boots with both the uboot-pinephone and tow-boot when booting from the sd card and emmc.

And since it seemed like there might be a little confusion, this doesn't replace uboot-pinephone with tow-boot by default, it just makes it so tow-boot can boot arch from the sd card.

[ArenM]

bump, is there anything blocking this?