kbuild: doc: describe proper script invocation

During an investigation to fix up the execute bits of scripts in the
kernel repository, Andrew Morton and Kees Cook pointed out that the
execute bit should not matter, and that build scripts cannot rely on that.
Kees could not point to any documentation, though.

Masahiro Yamada explained the convention of setting execute bits to make
it easier for manual script invocation.

Provide some basic documentation how the build shall invoke scripts, such
that the execute bits do not matter, and acknowledge that execute bits are
useful nonetheless.

This serves as reference for further clean-up patches in the future.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ujjwal Kumar <ujjwalkumar0501@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/lkml/20200830174409.c24c3f67addcce0cea9a9d4c@linux-foundation.org/
Link: https://lore.kernel.org/lkml/202008271102.FEB906C88@keescook/
Link: https://lore.kernel.org/linux-kbuild/CAK7LNAQdrvMkDA6ApDJCGr+5db8SiPo=G+p8EiOvnnGvEN80gA@mail.gmail.com/
Link: https://lkml.kernel.org/r/20201001075723.24246-1-lukas.bulwahn@gmail.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>