Use common interface to build linker configuration

Current filesystem logic is quite separated with systemimage, so it can
end up generating linker configuration twice which can end up an error
case. This change creates a common interface and let both filesystem and
systemimage can implement their own function to generate linker
configuration.

Bug: 324995772
Test: Compared linker.config.pb of generic_system_image with this patch
Change-Id: Ic515e54deeafbae74fd02bb023661606aa7e0b7c