bootstd: make it possible to use tftp for netboot with standardboot

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>

doc/develop/bootstd/script.rst

@@ -12,8 +12,10 @@ list of prefixes (``{"/", "/boot"}`` by default) and can be adjust with the
 `filename-prefixes` property in the bootstd device.
 
 For a network device, the filename is obtained from the `boot_script_dhcp`
-environment variable and the file is read using tftp. It must be in the
-top-level directory of the tftp server.
+environment variable. By setting the `ip_dyn` environment variable it can be
+decided if dynamic ip (dhcp command) or static ip (tftp command) is used for
+reading the file. By default dhcp is used. The file must be in the top-level
+directory of the tftp server.
 
 In either case (file or network), the bootmeth searches for the file and creates
 a bootflow if found. The bootmeth searches for "boot.scr.uimg" first, then