find_fcb_offset doesn't consider 4 as valid

[billr1]

The default offset is often 4, but thatg is rejected as invalid. Suggested patch below.
diff --git a/imxtools/init.py b/imxtools/init.py
index 72330d5..eb1f711 100644
--- a/imxtools/init.py
+++ b/imxtools/init.py
@@ -198,7 +198,7 @@ def find_fcb_offset(content):
"""
try:
index = content.index(b'FCB ')

•    if (index > 4):
  

•    if (index >= 4):
         return (index-4)
     return -1
  

  except ValueError as exc: