Flesh out a geom_map setup, so the kernel can be squeezed _onto_ the device.

The default flash layout gives only 1 megabyte for the kernel, gzipped.
The uboot firmware running on this device only supports gzip, not lzma, so
we actually _do_ have to try and slim the kernel down a bit.

But, since I can't actually do that at the present, I'm opting to:

• extend the kernel from 1mb to 2mb;
• have rootfs fill the rest of that, save 64k;
• eventually I'll hide a 64k config partition at the end, between the end of rootfs and the ART (radio configuration data.)

The uboot firmware doesn't care about the partition layout. It just
expects the kernel application image to sit at 0xbf020000 (right after
the 128k uboot image.) The uboot header isn't actually read either -
it's "faked" from a "tplink" flash image header. So as long as the
map configuration here matches what is being written out via the
tplink firmware generator, everything is a-ok.