Now that I've brought up FreeBSD via flash, I've discovered that
the second-last 64k seems to be the default firmware board configuration
area.

Since I have no idea whether uboot uses it or not - and it's prefixed
with an atheros eeprom signature (0xaa55), I figure the safest thing
to do is mark it as read-only.

I've modified my local tplink firmware building program to generate
a board configuration section - which is separate to this partition.
It's located in the 64k _before_ this particular 64k.

The firmware build program from OpenWRT never initialises those
values and the firmware images from tplink also leave it 0x0, so I
don't currently know what the exact, correct details should be.