Filter out lines starts with "//" from json file to allow simple
comments on the contents. Original json format does not support
comments, but this reduces readability compared to txt file or other
formats. This change allows simple comments on the linker.config.json to
give more information on the contents.
Test: parse succeeded with commented contents
Change-Id: I1c734bf9a054f81f57aa2aea1038d0041297acf1
Handle new filetype 'linker_config' which is configuration for
linkerconfig in json type and convert into protobuf in build time.
Bug: 169634881
Test: Build succeeded and cuttlefish boot succeeded
Change-Id: I56555fc738e6d6600d15a191a24f79a2ee747f52
