wl18xx_build: update for r8.a5.13 release

* Update the build script for fetching R8 SP1 release (r8.a5.13)
* Change the build script for creating the compat-wireless package
  directly from the github gits instead of fetching a manually made
  zip file from gforge
* Some code cleanup

Signed-off-by: Eyal Reizer <eyalr@ti.com>