Add the option to load the bootscript with the tftp command (static IP)
instead of the dhcp command (dynamic IP). For this a new function
tftpb_run similar to dhcp_run, is needed. The selection of which command
to use can be done with the ip_dyn environment variable, which can be
set to yes or no. The ip_dyn variable was chosen as it is already in use
on the imx platforms.
Also edit the bootstd doc.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>

Add documentation for the script bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>