Create writer and handle environment variable
To handle better with generating configuration and resolving variables,
new modules are introduced - ConfigWriter and Variables.
Bug:
123722631
Test: m -j && atest
Change-Id: Ibd44d5f93d566ce6d9bc04d474de83ec1e5e8ff4
Define modules for linker config
Define modules for linker config and implement config string generator
for each modules.
Bug:
123722631
Test: m -j && atest linkerconfig_modules_unittest
Change-Id: Id9a234830c381267c23ec7af657f354d51a58889