Shell: Improve `mkdir`

* Fix treating relative paths
* Add support for multiple paths
* Add option to auto create parent directories

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Shell: Add `cp` command

Currently supported flags:

* -R, -r, --recursive
* -t, --target-directory=DIRECTORY
* -T, --no-target-directory

Interactive not supported yet.

Closes #35.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>