classes/create-spdx: Add SPDX_PRETTY option

Adds an option to make the SPDX more human-readable (at the expense of a
larger files)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>