Unbreak zfsloader with LOADER_TFTP_SUPPORT on

Only accept 'net' and 'pxe' devices as underlying transport
in tftp.c on x86. Prior to this change tftp code would attempt
to send packets over any boot device, including zfs one with
predictably sad results.

Approved by: re (gjb)
MFC After: 1 month
Remove hash generation from release.sh, as it is run as part of
the 'install' target in the release/Makefile.

MFC after:	3 days
Submitted by:	jhb
Approved by:	re (hrs)
Sponsored by:	The FreeBSD Foundation
Add read/write bus barriers for MIPS.

Tested:

* AR9344 - mips74k SoC

Approved by:	re@ (delphij)