util/grub-editenv: Wire list_variables() to optional fs_envblk

util/grub-editenv: Wire list_variables() to optional fs_envblk

This patch updates list_variables() so that it also prints entries from
the external environment block when one is present. The function first
lists all variables from the file based envblk, then iterates over the
external envblk and prints those as well.

The output format remains the same as before. The change makes it
possible to inspect variables regardless of whether they are stored in
the file envblk or in the reserved block.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>