stand/kboot: Initialize all the devices

main() of the boot loader is expected to call devinit() early. We do
this at the same time we do it in the EFI loader (except we don't have a
buffer cache here, we don't need to initialize time and we don't have
special efi partition handles to enumerate). This is just after we probe
for the console.

Sponsored by: Netflix

(cherry picked from commit 299c64e316f026a9441775f21dfdb69e8125d58e)