Various fixes to PRODUCT_BUILD_<IMAGE> variables.

- Introduces PRODUCT_BUILD_VENDOR_BOOT_IMAGE.
  - Controls vendor_boot.img, replacing TARGET_NO_VENDOR_BOOT.
  - Matches the naming convention of other similar vars.
- Guards boot-debug.img behind BUILDING_BOOT_IMAGE
- Restructures BUILDING_BOOT_IMAGE to give priority to
  PRODUCT_BUILD_BOOT_IMAGE, as do other partitions.
- ^ for BUILDING_RECOVERY_IMAGE.

Test: PRODUCT_BUILD_{BOOT,RECOVERY,VENDOR_BOOT}_IMAGE := false
      m dist
      Observe no boot, boot-debug, recovery, or vendor_boot images.
Bug: 169968221
Bug: 170423509
Change-Id: I629bf08ba08e5db14c1bf92bb338fb3ce59d5b73