Fixes <http://bugs.gnu.org/22281>.
Reported by Christopher Allan Webber <cwebber@dustycloud.org>.

* gnu/system/grub.scm (eye-candy): Add 'root-fs' parameter.  Replace
'search --file' command in the output with whatever 'grub-root-search'
returns.
(grub-root-search): New procedure.
(grub-configuration-file): Add 'store-fs' parameter.  Use
'grub-root-search' instead of hard-coded 'search --file' commands.
* gnu/system.scm (store-file-system,
operating-system-store-file-system): New procedures.
(operating-system-grub.cfg): Use it, and adjust call to
'grub-configuration-file'.
* tests/system.scm: New file.
* Makefile.am (SCM_TESTS): Add it.