include: util: Add CONCAT as a general Zephyr utility macro

Provide the CONCAT macro as a general Zephyr utility macro
to paste two tokens during the preprocessor pass.

Note that this macro is based on the _CONCAT macro defined
in toolchain/common.h. This toolchain header needs a CONCAT
like macro, but requires minimal include dependencies.
So we leave _CONCAT where it is.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>