restore: fix access to ESP partition

Commit 29c7bb5 introduced assumption
that `mounts` has a `exp` key, which was not the case when called from
`restoreFromBackup`.  This makes sure the key is created.

Signed-off-by: Yann Dirson <yann.dirson@vates.tech>

Author: ydirson

restore.py

@@ -214,6 +214,7 @@ def restore_partitions():
             # restore boot loader
             if boot_config.src_fmt == 'grub2':
                 if efi_boot:
+                    mounts['esp'] = esp
                     backend.setEfiBootEntry(mounts, disk_device, boot_partnum, constants.INSTALL_TYPE_RESTORE, branding)
                 else:
                     if location == constants.BOOT_LOCATION_MBR: