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
wifi: add a new "loading" state

Set wlan.driver.status to "loading" in wifi_load_driver to allow
external action being triggered by property change.

Change-Id: Ia4363de4c4e0e1a1c4759f5699fd5ea6e1bc3182