wifi: add suport for WIFI_EXT_MODULE

Some wifi modules have dependencies on others (for SDIO, usually).
Use WIFI_EXT_MODULE_PATH to specify such dependencies

Change-Id: Ie8e746c1e69712be78d420d8e8ae8b015564a03d

Conflicts:

	wifi/wifi.c

Change-Id: I3fbc0065f76d91d170fb562159ea326caa4f3345