redoxfs: Add '--get-uuid' support

With this, `redoxfs --get-uuid <disk>` prints out the UUID of the
provided redoxfs disk image.

This is needed especially for aarch64 kernels where the UUID of the boot
disk needs to be passed to the kernel by the bootloader. With this
extension to the mount command, the kernel build glue can query the
prepared redoxfs image for it's UUID.
3 jobs for aarch64-prep in 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7847
build:linux:cargo

00:00:40

failed #7848
build:redox:cargo

00:00:32

 
  Test
skipped #7849
test:linux:cargo
 
Name Stage Failure
failed
build:redox:cargo Build
error: Could not compile `redoxfs`.

Caused by:
process didn't exit successfully: `rustc --crate-name redoxfs src/bin/mount.rs --color never --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=11a511198db319c0 -C extra-filename=-11a511198db319c0 --out-dir /builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps --target x86_64-unknown-redox -C linker=x86_64-unknown-redox-gcc -C incremental=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/incremental -L dependency=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps -L dependency=/builds/microcolonel/redoxfs/target/debug/deps --extern syscall=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps/libsyscall-f76fbb41bdc54636.rlib --extern redoxfs=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps/libredoxfs-8d94c1569bf84999.rlib --extern spin=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps/libspin-8076d0d3c6618c21.rlib --extern uuid=/builds/microcolonel/redoxfs/target/x86_64-unknown-redox/debug/deps/libuuid-9ef126a8fd8115e7.rlib` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
error: build failed
groupadd: group 'redox' already exists
useradd: user 'redox' already exists
ERROR: Job failed: exit code 1