stand/uboot: setup archsw before probing devices

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.

Reviewed by: imp, mw
Sponsored by: Stormshield
Obtained from: Semihalf
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35670