wifi: add suport for custom wifi.c

This patch adds TARGET_CUSTOM_WIFI which can be used to replace the standard wifi.c with a custom implementation without breaking things for devices that assume default functionality. https://github.com/pandigital/android_hardware_libhardware_legacy/commit/ded39b2117380340dd92c04504cda863b31ca217

Change-Id: Ic7cdce6c9e57b7619e0f9ea383c5ba4e47e2a129