commands/*: Replace license and copyright boilerplate by SPDX identfiers ...

While at it also drop references to the non-existing CREDITS file and do
some small rearrangements for some uniform formatting.
(SPDX-License-Identifier first, then copyright texts and then an empty
line.)

The advantage is that these specifiers are machine-parseable which helps
license conformance.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: implement 'cmp' command ...

This command compares two files. It does not show the differences,
it only returns successfully if both files are identical and with
an error if they differ.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>