Make update_verifier check if it runs on A/B update devices at the
beginning, and quit immediately if it doesn't, instead of re-boot.
Bug: 70541023
Test: On aosp/master:
$ lunch aosp_x86_64-userdebug; m -j # boot to home screen
# On goog/master:
$ lunch aosp_walleye-userdebug; m -j # boot to home screen
Change-Id: Ib71a3a3b272cfa5dd0b479eaa067eedaec8fde7d
Set up update_verifier logging to be written to kmsg; because we may
not have Logd during boot time.
Bug: 64713327
Test: logs show up in `adb shell dmesg`
Change-Id: If02f460bda121cd3e9062bc0e08107c6da66492c