OTA rollback test build config (sdkconfig.selftest_fail)

Builds an image whose self-test always fails, in its own build directory
(build-*/ ignored), for the step 2b rollback test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,4 @@ build/
 sdkconfig
 sdkconfig.old
 managed_components/
+build-*/

sdkconfig.selftest_fail

@@ -0,0 +1,4 @@
+# Rollback test build only (step 2b). Build in its own directory, e.g.:
+#   idf.py -B build-selftest-fail -DSDKCONFIG=build-selftest-fail/sdkconfig \
+#          -DSDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.selftest_fail" build
+CONFIG_AES67_OTA_SELFTEST_FORCE_FAIL=y