Add boot*.img to default dist target if PRODUCT_EXPORT_BOOT_IMAGE_TO_DIST

Right now if PRODUCT_EXPORT_BOOT_IMAGE_TO_DIST is given,
  $ m dist bootimage
would copy boot*.img to the dist dir.

After this change, both
  $ m dist  # droidcore
  $ m dist bootimage
would copy boot*.img to the dist dir, the difference is that the latter
builds & copies *only* the boot images.

Bug: 212486689
Test: m dist bootimage
Test: m dist => boot*.img are copied to dist dir
Change-Id: I1c714ce5d75eba93e32078b1ac291a181b05405a

core/main.mk

@@ -1877,6 +1877,10 @@ else ifeq ($(TARGET_BUILD_UNBUNDLED),$(TARGET_BUILD_UNBUNDLED_IMAGE))
     )
   endif
 
+  ifeq ($(PRODUCT_EXPORT_BOOT_IMAGE_TO_DIST),true)
+    $(call dist-for-goals, droidcore-unbundled, $(INSTALLED_BOOTIMAGE_TARGET))
+  endif
+
   ifeq ($(BOARD_USES_RECOVERY_AS_BOOT),true)
     $(call dist-for-goals, droidcore-unbundled, \
       $(recovery_ramdisk) \