gitignore: ignore multiple output directories

Developers may have multiple output directories when building multiple
boards in parallel. A previous commit attempted to gitignore new output
directories automatically but broke some things (commit a14c862c08
"Create a .gitignore file in the CANONICAL_O directory").

To provide some support, ignore anything in the Buildroot directory that
starts with "output". This is a bit more cumbersome, but doesn't break
packages based on Python maturin.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

.gitignore

@@ -1,4 +1,4 @@
-/output
+/output*
 /dl
 /.auto.deps
 /.config.cmd