imgtool: fix "align" script error

Fix "align" possible script error, caused by #1833.
Set default value for the align parameter.

Signed-off-by: Andrej Butok <[email protected]>

Author: butok

scripts/imgtool/main.py

@@ -377,6 +377,7 @@ def convert(self, value, param, ctx):
               'keyword to automatically generate it from the image version.')
 @click.option('-v', '--version', callback=validate_version,  required=True)
 @click.option('--align', type=click.Choice(['1', '2', '4', '8', '16', '32']),
+              default='1',
               required=False,
               help='Alignment used by swap update modes.')
 @click.option('--max-align', type=click.Choice(['8', '16', '32']),