Build tests: Use & document clean_build_dir

diff --git a/make/test_build_system.sh b/make/test_build_system.sh
index a6de14987be034396eb8801ffdab91652f0a4309..7c8cbc1f1eba8e74c563f4e3ecdf92d7108a8a3c 100755 (executable)
--- a/make/test_build_system.sh
+++ b/make/test_build_system.sh
@@ -21,7 +21,8 @@
 # * The function "take_build_snapshot" can be paired with the functions "assert_rebuilt" and "assert_not_rebuilt" to compare file timestamps and verify if they were rebuilt or not since the snapshot was taken.
 #
 # To add a new test case, add it to the end of the run_tests function. Note that not all test cases do comprehensive cleanup
-# (although very invasive ones like appending CRLFs to all files take a copy of the esp-idf tree.)
+# (although very invasive ones like appending CRLFs to all files take a copy of the esp-idf tree), however the clean_build_dir
+# function can be used to force-delete all files from the build output directory.
 
 # Set up some variables
 #
@@ -92,7 +93,7 @@ function run_tests()
     fi
 
     print_status "Bootloader build shouldn't leave build output anywhere else"
-    rm -rf --preserve-root ${BUILD}
+    clean_build_dir
     make bootloader
     # find wizardry: find any file not named sdkconfig.h that
     # isn't in the "bootloader" or "config" directories