wic: rm with -r flag support

wic currently unable to remove non-empty directory in ext* partition.
enable wic rm to remove non-empty directory and all the sub-content
with -r flag.
update help documents for 'wic rm'.
[YOCTO #12404]

(From OE-Core rev: 5cb7a329d0aaac8fe5328eb2001692c540aa5ade)

Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/wic

@@ -403,6 +403,9 @@ def wic_init_parser_rm(subparser):
                         help="path: <image>:<vfat partition><path>")
     subparser.add_argument("-n", "--native-sysroot",
                         help="path to the native sysroot containing the tools")
+    subparser.add_argument("-r", dest="recursive_delete", action="store_true", default=False,
+                        help="remove directories and their contents recursively, "
+                        " this only applies to ext* partition")
 
 def expandtype(rules):
     """