binman: Allow help to work without libfdt

At present binman needs libfdt.py to be available before it will do
anything, even print help. Import those modules later to avoid this, as it
is bad practice to fail to even show help on startup.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/control.py

@@ -13,8 +13,6 @@ import tools
 
 import command
 import elf
-import fdt
-import fdt_util
 from image import Image
 import tout
 
@@ -129,6 +127,11 @@ def Binman(options, args):
         options.indir.append(board_pathname)
 
     try:
+        # Import these here in case libfdt.py is not available, in which case
+        # the above help option still works.
+        import fdt
+        import fdt_util
+
         tout.Init(options.verbosity)
         elf.debug = options.debug
         try: