powerpc/configs/skiroot: Disable xmon default & enable reboot on panic

If the skiroot kernel crashes we don't want it sitting at an xmon
prompt forever. Instead it's more helpful to reboot and bring the
boot loader back up, and if the crash was transient we can then boot
successfully.

Similarly if we panic we should reboot, with a short timeout in case
someone is watching the console.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20200121043000.16212-8-mpe@ellerman.id.au