Add an option to multiplex AP and STA on wlan0

Add an option to multiplex AP and STA on wlan0

This adds CONFIG_MTK_COMBO_AOSP_TETHERING_SUPPORT which, when enabled,
allows ap and wlan to co-exist in the same interface, as Android
expects.

Most of this functionality is also available (albeit not compilable broken)
under CFG_TC1_FEATURE but that has larger implications around the radio
and usb stack that we do not want to adopt.

Change-Id: Ib1d1be40566f1bb9ccc7be45b49ec8d1f3b3ba58
Ticket: PORRIDGE-30