Builds boot-debug.img if INSTALLED_BOOTIMAGE_TARGET.

This causes boot-debug.img to build if its main dependency
INSTALLED_BOOTIMAGE_TARGET (boot.img) is defined.

This fixes an issue where boot-debug.img was missing when
BUILDING_BOOT_IMAGE=false, but boot.img exists because the board uses
recovery as boot.

Bug: 170644849
Test: Build target that sets PRODUCT_BUILD_BOOT_IMAGE and
      PRODUCT_BUILD_RECOVERY_IMAGE false,
      observe no boot-debug.img.
Test: Build target that sets BOARD_USES_RECOVERY_AS_BOOT,
      observe boot-debug.img.
Change-Id: Ic887ea93d4c5181eca0f82c3cdf3ce3b72f4c185