uboot: fix build for older uboot source trees

The change in commit bf733342 only works
for newer uboot source trees. Add a check that scripts/dtc/libfdt
directory exists before making this change.

[Peter: add comment explaining why]
Signed-off-by: Stefan Becker <chemobejk@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>