Add mk file parsers to support build variables required by system/bt

* BtConfigIncludeDir: Path to device specific config headers
                      BOARD_BLUETOOTH_BDROID_BUILDCFG_INCLUDE_DIR
* BTHcilpIncluded: Path to Hcilp headers
                   BOARD_BLUETOOTH_BDROID_HCILP_INCLUDED
* BtHciUseMct: Whether MCT is used
               BLUETOOTH_HCI_USE_MCT

Bug: 32958753
Test: Code compilation, no user visible effects

Change-Id: I94cad65c77464e5b467abee05951ddfd35b9f27d