Allow comments from linker.config.json

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