Move wifi.c to libwifi-system

This library will eventually contain only device independent wifi
related code.  For now, it still takes a few parameters from the build
system to control #defines in a device specific way.

Bug: 29321445
Change-Id: I6d0054a15b0e448019bb51728763d961132c5464
Test: Compiles, can associate to APs on bullhead, angler
10 files changed
tree: 06d86963d68097bc0c1bb6eebe8a8a563decce9f
  1. libwifi_system/
  2. service/
  3. tests/