doc: imgtool: update align description

Update the --align option values.
Add its description.
Delete [required], as it is optional now.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>

docs/imgtool.md

@@ -61,7 +61,7 @@ primary slot and adds a header and trailer that the bootloader is expecting:
     Options:
       -k, --key filename
       --public-key-format [hash|full]
-      --align [1|2|4|8]             [required]
+      --align [1|2|4|8|16|32]       Alignment used by swap update modes.
       -v, --version TEXT            [required]
       -s, --security-counter TEXT   Specify the value of security counter. Use
                                     the `auto` keyword to automatically generate

scripts/imgtool/main.py

@@ -377,7 +377,8 @@ class BasedIntParamType(click.ParamType):
               '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']),
-              required=False)
+              required=False,
+              help='Alignment used by swap update modes.')
 @click.option('--max-align', type=click.Choice(['8', '16', '32']),
               required=False,
               help='Maximum flash alignment. Set if flash alignment of the '