manual: do not generate .pyc files

Python saves a pre-compiled support/scripts/kconfiglib.pyc file
side-to-side with the corresponding .py file.

This does not work if the Buildroot source tree is read-only (but
this is not an error for Python, which keep going OK).

But this may cause issues for out-of-tree builds in case the same
Buildroot source tree is shared by many builds.

Also, 'make clean' currently does not clean this file, and out-of-tree
builds can remove it either, at the risk of causing issues for other
out-of-tree builds running at the same time.

Just tell Python not to generate .pyc files:
  - call the script via python, don't use the sha-bang
  - thus, make the script non-executable, and remove the sha-bang

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>