Suggested by John Darrington <john@darrington.wattle.id.au>.
* gnu/system/file-systems.scm (%not-slash): New variable.
(file-prefix?): New procedure.
(file-system-needed-for-boot?): Use it to check whether FS holds the
store.
* tests/file-systems.scm ("file-system-needed-for-boot?"): New test.
* gnu/tests/install.scm (%separate-store-os)[file-systems]: Remove
'needed-for-boot?' field for "/gnu".