Allow passing in `--overwrite-only` flag

Overwrite only requires just magic + image_ok + copy_done. This fixes
issues generating images in overwrite only mode when the firmware image
is too big and overflows the swap status area.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool.py b/scripts/imgtool.py
index f3bd016..ccac193 100755
--- a/scripts/imgtool.py
+++ b/scripts/imgtool.py
@@ -114,6 +114,8 @@
 
 @click.argument('outfile')
 @click.argument('infile')
+@click.option('--overwrite-only', default=False, is_flag=True,
+              help='Use overwrite-only instead of swap upgrades')
 @click.option('-M', '--max-sectors', type=int,
               help='When padding allow for this amount of sectors (defaults to 128)')
 @click.option('--pad', default=False, is_flag=True,
@@ -129,12 +131,13 @@
 @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,
-         max_sectors, infile, outfile):
+         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,
-                           max_sectors=max_sectors)
+                           max_sectors=max_sectors,
+                           overwrite_only=overwrite_only)
     key = load_key(key) if key else None
     img.sign(key)