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>

add of_diff command ...

The of_diff command compares two device trees against each other
and prints a diff-like result. This can be handy to find out the
differences between the barebox live tree and the one we start the
kernel with. Another usecase would be to examine the changes our
of_fixup process introduces (of_diff - +)

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