MFC r327048-r327050

MFC r327048-r327050

r327048:
Restore the ability to use EARLY_PRINTF support during most of initarm().

The real kernel page tables are set up much earlier in initarm() now than
they were when early printf support was first added, and they end up undoing
the mapping made in locore.S for early printf support.  This re-adds the
mapping after switching to the new/real kernel page tables, making early
printf work again right after switching to them.

r327049:
Allow pmap_kremove() to remove 1MB section mappings as well as 4K pages.
This will allow it to undo temporary device mappings such as those made
with pmap_preboot_map_attr().

Reviewed by:	cognet

r327050:
If a temporary mapping is made to support EARLY_PRINTF, undo that mapping
after cninit() runs, otherwise we leave a bogus device-memory mapping in
userspace VA in the kernel pmap forever.

Pointed out by:	cognet