gitignore: Only ignore build folder at the repo root

We want to ignore the "build/" folder created by tox when it builds an
sdist for its dev environment, but we do not want to ignore the
"build" folders further down the path hierarchy, as we have a
src/mbed_tools/build folder which contains source code we need git to
track.

Author: rwalton-arm

.gitignore

@@ -11,7 +11,7 @@ Pipfile.lock
 *.pyc
 __pycache__/
 *.egg-info/
-build/
+/build/
 
 # Coverage.py
 .coverage*