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
by replacing `0` with a number of seconds.
2 jobs for panic in 76 minutes and 49 seconds (queued for 23 seconds)
Status Job ID Name Coverage
  Build
passed #368398
librem5
build:aarch64

00:15:53

passed #368399
librem5:arm64
build:deb

01:16:45