Update README.md and HACKING.md

HACKING.md

@@ -15,3 +15,6 @@ There are several gradle flags one can use for Konan build.
 
         ./gradlew -Pfilter=overflowLong.kt run_external
 
+* **-Pperfix** allows one to choose external test directories to run. Only tests from directories with given prefix will be executed.
+
+        ./gradlew -Pfilter=external_codegen_box_cast run_external

README.md

@@ -22,4 +22,10 @@ For some tests, use:
 
 To run blackbox compiler tests from JVM Kotlin use (takes time):
 
-    ./gradlew run_external
+    ./gradlew run_external
+
+To update the blackbox compiler tests set TeamCity build number in `gradle.properties`:
+
+    testDataVersion=<build number>:id
+
+and run `./gradlew update_external_tests`

backend.native/tests/build.gradle

@@ -44,6 +44,7 @@ allprojects {
     }
 }
 
+// TODO: use lastSuccessful teamcity build.
 task update_external_tests() {
     doLast {
         // Unzip.