Add proper consinit(9) support for sti(4) at sgc framebuffer on hp300.

The cnattach functions for sti(4) and service switch check method
for 425e in com_frodo.c are taken from OpenBSD.
The strategy how to choose the console device in hp300_cninit() is
quite diverged from 4.4BSD and OpenBSD so it's tweaked by me.

Also put several changes in sti_sgc.c to reduce diffs from OpenBSD/hp300.

Tested on 425e and 362 (which still uses gendiofb(4), not sti(4)).

XXX: sti(4) requires uvm_km_alloc(9) and uvm_map_protect(9)
     to copy and call ROM functions on the executable memory region, so
     it can be called before UVM and related initializations are complete.
     Probably it's time to consider about MI "deferred consinit()" API
     in init_main.c (or elsewhere) for modern complicated VM system...
Fetch the ethernet address from u-boot boardinfo and provide it as mac-address
property for ethernet controllers.
Mark /rescue/ldconfig as 'pic' only to enable building static releases (NOPIC=).
Only MKX11=no tested.
include cpufunc_proto.h since things need it.