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_
This changes the configuration macro to be the exponent of 2^n, as the
mbox buffer size needs to be always power of 2.
Also a compile configuration documentation group is created.