Move part of (gnu build linux-boot) to (gnu build file-systems).

Move part of (gnu build linux-boot) to (gnu build file-systems).

* gnu/build/linux-boot.scm (%ext2-endianness, %ext2-sblock-magic,
  %ext2-sblock-creator-os, %ext2-sblock-uuid, %ext2-sblock-volume-name,
  read-ext2-superblock, ext2-superblock-uuid,
  ext2-superblock-volume-name, disk-partitions,
  partition-label-predicate, find-partition-by-label,
  canonicalize-device-spec, MS_RDONLY, MS_NOSUID, MS_NODEV, MS_NOEXEC,
  MS_BIND, MS_MOVE, bind-mount, check-file-system,
  mount-flags->bit-mask, mount-file-system): Move to...
* gnu/build/file-systems.scm: ... here.  New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
* gnu/services/base.scm: Use (gnu build file-systems).
* gnu/services/dmd.scm (dmd-configuration-file): Likewise.
* gnu/system.scm (operating-system-activation-script): Likewise.
* gnu/system/linux-initrd.scm (base-initrd): Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Likewise.