bootstat: kernel_panic test w/o bootloader or pstore support

If the platform has no bootloader or pstore support, kernel_panic
test should fail if the results are not correct. Drop skipping of
failed test if pstore support is lacking.

If device demonstrably has pstore content support, the result must
be exacting kernel_panic,sysrq. Otherwise accept the less precise
result.

Test: On hikey960 (which currently lacks reliable pstore, or a
      compliant bootloader reporting bootreason), expect failure of:
      system/core/bootstat/boot_reason_test.sh kernel_panic
Bug: 63736262
Change-Id: I071a2a9c00dc522ec037c8a8997fea524d17e6e4