guix system: Check for the lack of modules in the initrd.

guix system: Check for the lack of modules in the initrd.

* guix/scripts/system.scm (check-mapped-devices): Take an OS instead of
a list of <mapped-device>.  Pass #:needed-for-boot? and #:initrd-modules
to CHECK.
(check-initrd-modules): New procedure.
(perform-action): Move 'check-mapped-devices' call first.  Add call to
'check-initrd-modules'.
* gnu/system/mapped-devices.scm (check-device-initrd-modules): New
procedure.
(check-luks-device): Add #:initrd-modules and #:needed-for-boot?.  Use
them to call 'check-device-initrd-modules'.