wifi: Add option for a device-specific wifi

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. (Derived from
Gingerbread:566d2f2)

Change-Id: Ief8c51c4c33f879c7771af8769c83f1f89979034
wifi: add new flag PRESTO_WIFI

Change-Id: Id6991026d4da8330488c1185470492eca3ec6a23