This changes the prefixes of the symbols generated from USEMODULE and
USEPKG variables. The changes are as follow:
KCONFIG_MODULE_ => KCONFIG_USEMODULE_
KCONFIG_PKG_ => KCONFIG_USEPKG_
MODULE_ => USEMODULE_
PKG_ => USEPKG_
Added a new section, Appendix D, to describe a few key aspects
while exposing a macro to Kconfig.
Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
This adds information regarding the usage of Kconfig from an user
perspective and in-depth information on how Kconfig is currently
integrated to RIOT's build system.
