HEADS UP: Bootloader changes

[Nathan Whitehorn]

If you use FreeBSD/ppc 12-CURRENT, make sure to either update world at 
the same time as the kernel or before it when upgrading past r326220 
(today). The interface between loader(8) and the kernel has changed, 
with the result that you will get to a mountroot prompt with a new 
kernel and an old loader. If you somehow do end up there, entering the 
root partition at the mountroot prompt will let things continue so that 
you can upgrade world.
-Nathan