shelldriver: Fix xmodem

xmodem was failing because it would consume the trailing newline
character between the `echo` for the marker and the command output. Fix
this by suppressing new line on the marker

Signed-off-by: Joshua Watt <Joshua.Watt@garmin.com>
Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: JoshuaWatt

labgrid/driver/shelldriver.py

@@ -314,7 +314,7 @@ def _start_xmodem_transfer(self, cmd):
         """
 
         marker = gen_marker()
-        marked_cmd = f"echo '{marker[:4]}''{marker[4:]}'; {cmd}"
+        marked_cmd = f"echo -n '{marker[:4]}''{marker[4:]}'; {cmd}"
         self.console.sendline(marked_cmd)
         self.console.expect(marker, timeout=30)