bootctl: add new --print-root-device option

We already have this nice code in system that determines the block
device backing the root file system, but it's only used internally in
systemd-gpt-generator. Let's make this more accessible and expose it
directly in bootctl.

It doesn't fit immediately into the topic of bootctl, but I think it's
close enough and behaves very similar to the existing "bootctl
--print-boot-path" and "--print-esp-path" tools.

If --print-root-device (or -R) is specified once, will show the block device
backing the root fs, and if specified twice (probably easier: -RR) it
will show the whole block device that block device belongs to in case it
is a partition block device.

Suggested use:

        # cfdisk `bootctl -RR`

To get access to the partition table, behind the OS install, for
whatever it might be.

test/test-bootctl-json.sh

@@ -22,3 +22,20 @@ command -v jq >/dev/null || {
 
 "$bootctl" list --json=pretty | jq . >/dev/null
 "$bootctl" list --json=short | jq . >/dev/null
+
+# bootctl --print-root-device should either succeed or fail with exit status 80
+# (because not backed by a single block device), but not fail otherwise.
+"$bootctl" -R || test "$?" -eq 80
+"$bootctl" -RR || test "$?" -eq 80
+
+if "$bootctl" -R > /dev/null ; then
+    P=$("$bootctl" -R)
+    PP=$("$bootctl" -RR)
+
+    echo "$P vs $PP"
+    test -b "$P"
+    test -b "$PP"
+
+    # $P must be a prefix of $PP
+    [[ $P = $PP* ]]
+fi