Add laptop-as-TFTP-server and XMODEM console upgrade methods

- Method 3: laptop with console + Ethernet can act as temporary TFTP server
- Method 4: XMODEM over console cable when USB broken and no Ethernet available
- Note 3-hour transfer time warning for XMODEM at 9600 baud

https://claude.ai/code/session_01JR2EMK7rwrZJowpstcaxQ6

README.md

@@ -544,18 +544,49 @@ The switch reboots after the agent upgrade.
 
 ---
 
-### Method 3 — TFTP (requires TFTP server on management computer)
+### Method 3 — TFTP from a laptop (no dedicated server needed)
 
-Install a TFTP server (`sudo apt install tftpd-hpa`), place the firmware files in `/srv/tftp/`, then from enable mode:
+Your laptop can act as a temporary TFTP server. Plug both the console cable **and** an Ethernet cable from the laptop into the switch at the same time.
 
+On the laptop:
+```bash
+sudo apt install tftpd-hpa
+sudo cp ers5900diag_*.bin ers5900_*.img /srv/tftp/
+sudo ip addr add 192.168.1.50/24 dev eth0
+sudo systemctl start tftpd-hpa
 ```
-copy tftp address 192.168.99.50 filename ers5900diag_7x_x_x_x.bin
+
+Then on the switch via console:
+```
+enable
+copy tftp address 192.168.1.50 filename ers5900diag_7x_x_x_x.bin
 ```
 Then:
 ```
-copy tftp address 192.168.99.50 filename ers5900_7x_x_x_x.img
+copy tftp address 192.168.1.50 filename ers5900_7x_x_x_x.img
 ```
 
+Stop the server when done:
+```bash
+sudo systemctl stop tftpd-hpa
+```
+
+---
+
+### Method 4 — XMODEM over console cable (last resort, no network required)
+
+If the USB port is broken and you have no Ethernet available at all, boot menu option 4 ("Download Agent/Diag") supports XMODEM file transfer directly over the serial console cable. No network required.
+
+**Warning:** At 9600 baud, a 10 MB firmware file takes approximately 3 hours. Only use this if nothing else is possible.
+
+In `screen`, after selecting option 4 from the boot menu, send the file with:
+
+```
+Ctrl-A  then  :exec !! sx -b /path/to/ers5900_7x_x_x_x.img
+```
+
+(`sx` is part of the `lrzsz` package: `sudo apt install lrzsz`)
+
 ---
 
 ## Troubleshooting