github: add codecov.yml

Don't report until both cov runs are available to avoid partial
reporting in pull request comments.

.github/codecov.yml

@@ -0,0 +1,22 @@
+codecov:
+  require_ci_to_pass: yes
+  notify:
+    after_n_builds: 2
+
+coverage:
+  precision: 2
+  round: down
+  range: "70...100"
+
+parsers:
+  gcov:
+    branch_detection:
+      conditional: yes
+      loop: yes
+      method: no
+      macro: no
+
+comment:
+  layout: "diff,flags"
+  behavior: default
+  require_changes: no