DISKLABEL_EI option for system with MBR

[Rin Okuyama <rokuyama%rk.phys.keio.ac.jp@localhost>]

Hi,

I'd like to add DISKLABEL_EI kernel option, cf tech-kern/51208:

  http://gnats.netbsd.org/51208

Currently, we have FFS_EI and LFS_EI kernel options, that enable us to
mount UFS partitions written in the different byte order. However,
unfortunately, the system does not recognize disklabels written in the
different byte order.

The patch attached to tech-kern/51208 adds DISKLABEL_EI option for ports
using MBR. By this option, the system can mount the disk (image) written
in the different byte order. This should be useful for cross building.
For bi-endian architectures, like evbarm, the little and big endian
partitions can share a same disk by this option.

The patch seems almost trivial; if we encounter the disklabel written in
the different byte order, swap its order. The main patch is only ~250
lines including the manpage (The PR is very long since it also contains
the patch for kernel configuration files; it adds DISKLABEL_EI option
commented out except for ALL on amd64 and i386). Currently, this option
is restricted to ports with MBR. However, porting it to other ports may
also be trivial if someone needs.

I'd like to commit this patch if there's no objection (and enable it for
GENERIC kernels on i386 and amd64). Any comments?

Thanks,
rin