The previous sync was commit 18895514dc5e ("kconfig: update to
v3.18-rc6").

Some commits were applied on top of that since then, but equivalent
fixups were done in Linux.

So, this commit overwrites everything scripts/kconfig/ with the one
from Linux 4.20

Highlights:

 - 'silentoldconfig' has been renamed to 'syncconfig'
   (the top Makefile needs to be adjusted)

 - 'testconfig' target has been added for unit-tests
   (the top Makefile needs to export PYTHON3 because
    the test frame relies on Python3 and pytest)

 - The perfect hash table generated by gperf has been removed

 - The localization support has been removed

 - The 'option env=VAR' has been replaced with more generic
   variable reference syntax $(VAR)
   (./Kconfig and common/Kconfig need to be adjusted)

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>