Documentation/gen_commands.py use to re-write all auto-generated *.rst
file, changed or not. That in turn didn't work well with the internal
cache of the Sphinx documentation generator.

By comparing the SHA1 hash of the newly generated *.rst with the
current sha1 file, the time to execute "make docs" can be reduced
from 6.2s to 2.4s on my humble laptop.

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