Fix sonic-installer failure due to missing import (sonic-net#2090)

What I did
Fixes sonic-net#2089.
Sonic-installer failure was seen due to missing import in uboot.py.

How I did it
Fix is just adding missing import.

How to verify it
Fix has been tested for successful sonic installation from sonic-installer

Change-Id: I44bea3d2f89413d2d0dacfb1b6b7d19b15e52a09
Signed-off-by: Pavan Naregundi <[email protected]>

Author: pavannaregundi

sonic_installer/bootloader/uboot.py

@@ -4,6 +4,7 @@
 
 import platform
 import subprocess
+import os
 
 import click