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.

src/boot/bootctl.h

@@ -24,6 +24,7 @@ extern char *arg_esp_path;
 extern char *arg_xbootldr_path;
 extern bool arg_print_esp_path;
 extern bool arg_print_dollar_boot_path;
+extern unsigned arg_print_root_device;
 extern bool arg_touch_variables;
 extern PagerFlags arg_pager_flags;
 extern bool arg_graceful;