MFC r305094, r305096-r305097

r305094 by cognet:
Garbage collect bits forgotten in r295267.

r305096 by cognet:
Some old arm ports don't load the kernel at the beginning of the memory,
because the bootloader, ie redboot, won't let them do so, and so used the
memory before the kernel for early memory allocation, such as pagetables,
stacks, etc...
Make a bit of an effort to try to get that memory mapped.

r305097 by cognet:
Nuke obio_bs_tag, it was used before it was initialized, and
arm_base_bs_tag is the same, anyway.