devtool: build-image: rename module

Hyphens aren't allowed in python identifiers, so you shouldn't use them
in module names or they are more difficult to import.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

diff --git a/scripts/lib/devtool/build-image.py b/scripts/lib/devtool/build_image.py
similarity index 100%
rename from scripts/lib/devtool/build-image.py
rename to scripts/lib/devtool/build_image.py