scripts: kconfig: Replace Kconfig exectuables with Python

We have been using a fork of the Linux kernel's Kconfig system to
configure the Zephyr tree. The issue is that this is a native tool
written in C that is not easy to compile for Windows. This patch
replaces the use of the conf executable with kconfig.py, a script that
uses Kconfiglib to generate the .config and autoconf.h files required to
compile Zephyr.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>