libbt: Add btlock support

 * Concurrent WiFi/BT firmware loading can cause a chip crash.
 * Kernel supports a btlock device which is a simple semaphore that can
   protect from this situation.
 * Add support in the Bluedroid vendor lib for this on the BT side, as
   WiFi is already done directly in the kernel.

Change-Id: I8b9d3b1ccd06cc5d5528c7d6d685048c258d9887
libbt: Add prepatch support

 * We need to apply an initial patch to the BCM4335 hardware to prevent
   chip crashes, so add support for a prepatch designated by
   FW_PRE_PATCH.

Change-Id: I2d2ec59094a1ae2cb39e1f41d4853ac4a45d1902
libbt: Add support for using two stop bits

 * This improves reliability on chips such as BCM4335.
 * Also externalize some configuration.

Change-Id: I1271f0c1c3e602c8bfe9f816f57c9cb6add04b3a