bootstat: refine kernel panic details

If bootloader reports kernel_panic, gives us permission for a
deeper check of which kind occurred.  Resolve a problem with
some kernels that do not have the 'sysrq: SysRq :' log stutter.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: Idb8978de7a4f4e3c671c6ab7efc5ebe841f7bbd8