missing: use btrfs.h instead of defining our own btrfs structures
gpt-auto-generator: add basic auto-discovery of GPT partitions

This adds a simple generator that is capable of automatically
discovering certain GPT partitions by their type UUID and mount/enable
them. This currently covers swap partitions and /home partitions, but is
expected to grow more features soon.

This currently doesn't handle LUKS encrypted /home.

This enables all swap partitions of type
0657fd6da4ab43c484e50933c84b4f4f, if found.

This mounts the first partition of type 933ac7e12eb44f13b8440e14e2aef915
as /home, if it is found.
gpt-auto-generator: Skip /home mounting if /home is not empty
build-sys: add two makefile symlinks