Build OTA when boot image exists even without kernel or recovery fstab

On devices with a prebuilt boot image, TARGET_NO_KERNEL
may be set to enable signing, etc. In this case we still
want to build the OTA package.

Test: m otapackage on a device with generic boot image
      (where TARGET_NO_KERNEL is set)
Bug: 193588301
Change-Id: I4e5adc3f42a516ac0e2f66c313dbe34a469ebe05
Merged-In: I4e5adc3f42a516ac0e2f66c313dbe34a469ebe05