system: Record store file system info in each generation.

* gnu/system.scm (<boot-parameters>)[store-device, store-mount-point]:
New fields.
(read-boot-parameters): Initialize them.
(operating-system-grub.cfg): Likewise.  Remove STORE-FS argument from
call to 'grub-configuration-file'.
(operating-system-parameters-file): Add 'store' element in
'boot-parameters'.
* gnu/system/grub.scm (strip-mount-point): Replace 'store-fs' parameter
by 'mount-point'; adjust accordingly.  Adjust callers.
(<menu-entry>)[device, device-mount-point]: New fields.
(eye-candy): Replace 'root-fs' parameter by 'store-device'; add
'store-mount-point'.  Use keyword arguments for 'system' and 'port'.
(grub-root-search): Remove 'root-fs' by 'device' and adjust
accordingly.
(grub-configuration-file): Remove 'store-fs' parameter.  Adjust
accordingly.
* guix/scripts/system.scm (previous-grub-entries): Initialize 'device'
and 'device-mount-point' fields from PARAMS.
* doc/guix.texi (GRUB Configuration): Document 'device' and
'device-mount-point'.  Explain that 'linux' can be prefixed by a GRUB
device name.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>