cmake: Zephyr module processing set modules path

This commit introduces the variable:
ZEPHYR_${MODULE}_MODULE_DIR that can be used for modules to obtain
locations of other modules.

As example: a module which requires the knowledge of mcuboot con now
fetch this information using ZEPHYR_MCUBOOT_MODULE_DIR.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>