Soong: Add mk2bp conversion for prebuilt_firmare.

There are a couple of makefiles that have etc with $(TARGET_OUT)/etc/firmware,
$(TARGET_OUT_VENDOR)/firmare as the local module path. Added the conversion in
androidmk for target.

Bug: b/133711895
Test: Unit test cases.
Change-Id: I74aca18a7eb8aa4dd692cde680fa4e112edcf6d1