Add BOARD_BUILD_SUPER_IMAGE_BY_DEFAULT flag.

If set to true,
- super.img is built from images in $PRODUCT_OUT
- super.img is built to $PRODUCT_OUT
- super.img is built when 'make'.

'make dist' still builds super image from images in target files to
intermediates directory.

This flag is useful for virtual devices, but isn't intended to be set for
actual devices. For actual devices, userspace fastboot should flash
super_empty.img and individual user images separately.

Test: on cuttlefish (flag is set),
      make            # generates $OUT/super.img
      make superimage # generates $OUT/super.img
Test: on real non-retrofit DAP device (flag is not set)
      make            # no super.img is generated
      make superimage # generates $OUT/obj/PACKAGING/.../super.img
      make dist       # generates out/dist/super.img

Bug: 113175337

Change-Id: Ieb81e4fbb663bb4b69e9962c3fa9f16b03aeb907