file-systems: Always wait for devices to show up.

Previously, 'canonicalize-device-spec' would wait for devices when they
were specified as a label or UUID, but would not wait when the user
passed a "/dev" file name directly.  This could cause problems when
the /dev node takes a while to show up.

* gnu/build/file-systems.scm (canonicalize-device-spec): Add 'resolve'
call in the 'string?' case.

gnu/build/file-systems.scm

@@ -499,8 +499,8 @@ were found."
 
   (match spec
     ((? string?)
-     ;; Nothing to do.
-     spec)
+     ;; Nothing to do, but wait until SPEC shows up.
+     (resolve identity spec identity))
     ((? file-system-label?)
      ;; Resolve the label.
      (resolve find-partition-by-label