add codecov yaml configuration file

Adjust precision and range for labgrid needs. Smaller fixes often only cover a
few lines, therefore decrease the precision.

Most things in labgrid are not easily testable, a range from 50..70 should be
okay as a starting point.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>

.codecov.yaml

@@ -0,0 +1,3 @@
+coverage:
+  precision: 1
+  range: "50..70"