Depend on the ramdisk/root files for BOARD_BUILD_SYSTEM_ROOT_IMAGE

When using BOARD_BUILD_SYSTEM_ROOT_IMAGE, there is no direct dependency
on the files that get typically added to the ramdisk.img apart from the
NOTICE.html dependency that some files have. This change explicitly adds
the ramdisk/root files as a dependencies of system.img so that the image
is recreated any time a file in the ramdisk changes. It also creates the
installed-files-root.txt file.

Bug: 62387674
Test: /init.rc is present in system.img's dependencies
Change-Id: I234cb8d8a648f8963610be519de1b70b7efe1fd7