librem5_defconfig: Set CONFIG_PANIC_TIMEOUT to reboot immediately
The default behavior on kernel panic is to hang indefinitely, which isn't great for a battery powered device where investigating a panic after it happened may not even be feasible. Therefore, set up the kernel so it reboots on panic.
For debugging, this can be disabled at runtime by calling
sysctl kernel.panic=0. A delay before rebooting can be set
0 with a number of seconds.