kconfig: Remove targets specific to the C implementation

They will no longer be available once the C Kconfig implementation is
removed.

oldconfig and allno/yesconfig implementations are available for
kconfiglib and could be added later if needed. savedefconfig (minimal
configuration generation) is available from the menuconfig.py
configuration interface.

cmake/usage/kconfig-usage.cmake becomes kinda pointless after this
change, so merge it into cmake/usage/usage.cmake.

Remove the kconfig_target, COMMAND_FOR_*, and COMMAND_RUNS_ON_WIN_*
CMake variables, as there's just the 'menuconfig' target now.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>