Fix prebuilt boot image hermetic build

Break when we find a working salt, certain devices have boot image
containing both the kernel and ramdisk, resulting in the ramdisk being
used as salt. Since core/Makefile uses kernel as salt, update python
code to match.

Bug: 293313353
Test: th
Change-Id: I68b5a8c2f0747ddf68f775bc2a6c5db46eb7eb64