[armhf][sonic-installer] Fix the issue of sonic-installer list after …

…the set-default and cleanup

Signed-off-by: mlok <marty.lok@nokia.com>

sonic_installer/bootloader/uboot.py

@@ -40,7 +40,7 @@ def get_next_image(self):
         proc = subprocess.Popen("/usr/bin/fw_printenv -n boot_next", shell=True, text=True, stdout=subprocess.PIPE)
         (out, _) = proc.communicate()
         image = out.rstrip()
-        if "sonic_image_2" in image:
+        if "sonic_image_2" in image and len(images) == 2:
             next_image_index = 1
         else:
             next_image_index = 0