Update `--included-header` parameter

The `--included-header` was "mandatory" when using imgtool with firmware
images generated by the Zephyr build system and it was a source of
issues when it was forgotten. This removes `--included-header` and adds
a new parameter `--pad-header` with inverted semantics, to be used only
when a zeroed header is required to be added to the firmware image.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool.py b/scripts/imgtool.py
index ccac193..6531499 100755
--- a/scripts/imgtool.py
+++ b/scripts/imgtool.py
@@ -122,20 +122,19 @@
               help='Pad image to --slot-size bytes, adding trailer magic')
 @click.option('-S', '--slot-size', type=BasedIntParamType(), required=True,
               help='Size of the slot where the image will be written')
-@click.option('--included-header', default=False, is_flag=True,
-              help='Image has gap for header')
+@click.option('--pad-header', default=False, is_flag=True,
+              help='Add --header-size zeroed bytes at the beginning of the image')
 @click.option('-H', '--header-size', type=BasedIntParamType(), required=True)
 @click.option('-v', '--version', callback=validate_version,  required=True)
 @click.option('--align', type=click.Choice(['1', '2', '4', '8']),
               required=True)
 @click.option('-k', '--key', metavar='filename')
 @click.command(help='Create a signed or unsigned image')
-def sign(key, align, version, header_size, included_header, slot_size, pad,
+def sign(key, align, version, header_size, pad_header, slot_size, pad,
          max_sectors, overwrite_only, infile, outfile):
     img = image.Image.load(infile, version=decode_version(version),
-                           header_size=header_size,
-                           included_header=included_header, pad=pad,
-                           align=int(align), slot_size=slot_size,
+                           header_size=header_size, pad_header=pad_header,
+                           pad=pad, align=int(align), slot_size=slot_size,
                            max_sectors=max_sectors,
                            overwrite_only=overwrite_only)
     key = load_key(key) if key else None