test: spl: Add a test for the NET load method

test: spl: Add a test for the NET load method

Add a test for loading U-Boot over TFTP. As with other sandbox net
routines, we need to initialize our packets manually since things like
net_set_ether and net_set_udp_header always use "our" addresses. We use
BOOTP instead of DHCP, since DHCP has a tag/length-based format which is
harder to parse. Our TFTP implementation doesn't define as many constants
as I'd like, so I create some here. Note that the TFTP block size is
one-based, but offsets are zero-based.

In order to avoid address errors, we need to set up/define some additional
address information settings. dram_init_banksize would be a good candidate
for settig up bi_dram, but it gets called too late in board_init_r.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>