system: Add btrfs file system support.
* gnu/build/file-systems.scm (%btrfs-endianness, btrfs-superblock?,
read-btrfs-superblock, btrfs-superblock-uuid,
btrfs-superblock-volume-name, check-btrfs-file-system): New variables.
(%paritition-label-readers, %partition-uuid-readers): Add btrfs
readers.
* gnu/system/linux-initrd.scm (linux-modules): Add btrfs modules when a
btrfs file-system is used.
* gnu/tests/install.scm (%btrfs-root-os %btrfs-root-os-source,
%btrfs-root-installation-script, %test-btrfs-root-os): New system
test.
* doc/guix.texi: Adjust accordingly.
Fixes <http://bugs.gnu.org/19280>.
b1a505baf6