vendor/etc/linker.config.pb from multiple input files

When we have dedicated .mk files for each modules (e.g. usb, drm, etc)
and those modules have their own linker configuration requirements, it
would make more sense to have "fragments" for linker configuration.

This change introduces a new list variable to store the list of linker
configuration fragments. When it's set, vendor/etc/linker.config.pb is
generated from the list of input fragments.

Bug: 264330513
Test: set PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS
Test: m vendorimage (generates vendor/etc/linker.config.pb)
Change-Id: I9eed0f90add0191885b7195efdab94b5b1a4a62d