Support to build image of root file system with /system and ramdisk combined.

Added support to build system.img that combines contents of /system and
the ramdisk, and can be mounted at the root of the file system.
To enable this feature, define BoardConfig.mk variable:
BOARD_BUILD_SYSTEM_ROOT_IMAGE := true

Ideally we would just change TARGET_OUT (the path of the staging system
directory) to under TARGET_ROOT_OUT. But at this point many places in
the build system assume TARGET_OUT is independent of TARGET_ROOT_OUT and
we can't make it easily configurable.
Instead this implementation takes the least intrusive approach:
We don't change TARGET_OUT or TARGET_ROOT_OUT. We just assemble a
temporary staging directory that contains contents of both TARGET_OUT
and TARGET_ROOT_OUT, in build_image.BuildImage() of
tools/releasetools/build_image.py.
When build_image.py is directly called from the makefile, we pass in the
parameters from the global dictionary; when build_image.BuildImage() is
called from add_img_to_target_files.py, we need to override values to
point to files extracted from the target_files zip file.
We need to combine the fs_config files of both /system and ramdisk,
when fs_config is enabled.

Also this change refactored build_image.BuildImage() by moving the extra
parameters to the image property dictionary.

(cherry-picked from commit 0eabd4f2)

Bug:19868522
Change-Id: Iafc467a0e3427b0d6ad3b575abcc98ddcc9ea0f1