Also extract kernel configs from boot image.

On devices with prebuilt generic boot image:
  TARGET_NO_KERNEL := true
  BOARD_PREBUILT_BOOTIMAGE is set
In this case, the process for extracting the kernel
config and version must also try extracting from the
prebuilt boot image.

Test: build on device with prebuilt boot image
Fixes: 193588301
Change-Id: I43dd8e206bcd1c9f3f5f51869f126791c50af01a
Merged-In: I43dd8e206bcd1c9f3f5f51869f126791c50af01a