Enable building Phosh image for Librem 11

Enable building an image for Librem 11, with PureOS crimson, Phosh, and the Librem 11-specific base packages.

Much of this should be suitable for the Librem 5 as well and was based on existing work in Librem5/image-builder.

The build script now accepts new parameter values:

• Environment = librem-11 - installs librem11-base, etc. to bring in Phosh and Librem 11-specific config.
• Image style = flash - build a disk image for direct writing to the target system disk, with resize and reencryption on first boot