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