Librem 5 support

[dos1]

This is very obviously not mergeable and I probably won't work on it anymore, but it might be a good base for someone to take over and write proper support, so sharing it here.

Instead of producing SD/eMMC image like for PinePhone, this produces an initramfs that can then be booted straight via USB by using uuu.

What's left to do:

• building u-boot with https://source.puri.sm/Librem5/u-boot-builder/
• building kernel tree from https://source.puri.sm/Librem5/linux-next/
• a kernel defconfig that doesn't need modules for USB and SD card reader to function
• better splash images

[asriel-danctnix]

I don't have a Librem5 but it's cool to see non-Pine64 device running Jumpdrive.

[dos1]

So turns out I actually went forward and put it into shape that hopefully allows it to get merged :)

[MartijnBraam]

Oh cool, seems mostly allright.

Maybe have the L5 target output a .tar.gz containing the required files and a shell script to launch uuu so that can be main target to build this and to have a file to redistribute.

edit: no purism logo etc in the splash screen?

[craftyguy]

I think the dtb here should be the one for Birch, since it'll work on r2, r3, r4, right? If this uses the r4 dtb, then it'll only work on Evergreen?

[dos1]

It appears @craftyguy got tired of waiting for me and implemented your suggestions by himself - so I just included his branch in this MR (plus some minor adjustments from me).

The only thing left would then be the splash, which - honestly - IMO looks fine the way it is. Feel free to change it if you want though ;)