stand/uboot: setup archsw before probing devices

[aja_semihalf.com]

Summary

In some cases ubldr would try to mount a disk device before
the archsw struct was filled with functions pointers.
This would result in a NULL pointer derefrence of the arch_getdev field.
Fix that filling the archsw functions earlier.
Note that this matches the EFI behavoiur.

[imp]

Haven't chased down all the ways this scenario might happen, but this change won't hurt and could help.

[mw]

stand/uboot/main.c

489

This is an unrelated change, and imo should be dropped when committting. Otherwise, LGTM.