android_system_image that generates linker.config.pb

android_system_image module type is a specialization of the
android_filesystem module type. Currently, it adds a build rule for
creating linker.config.pb from the information about all the other files
in the filesystem and includes linker.config.pb to the filesystem as
well.

To do so, the filesystem module now provides a function pointer which
subtype modules like android_system_image can implement to pass extra
files that they want to package in the filesystem.

In addition, the linkerconfig package is revised to make it possible to
build linker.config.pb file outside of the package.

Bug: 185391776
Test: m microdroid and inspect etc/linker.config.pb in it.

Change-Id: Id89c40b519213062860d7306029b8413d8d36a2d