Enable luks support

Using '-l' will encrypt the root partition with the given password

This needs

• vmdeboostrap!2 (merged)
• librem5-apps/osk-sdl!1 (merged)
• more on device testing

./build-image -b devkit -d amber-phone -l 1234567

Note that this breaks resizing the rootfs on boot. It's basically there to make testing encrypted images simpler.

Closes: #57 (closed)