Rename guess_mbedtls_root to guess_project_root

Rename for consistency. Also, replace all calls to
this function with correct name.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/scripts/generate_ssl_debug_helpers.py b/scripts/generate_ssl_debug_helpers.py
index a0544f1..af8ef86 100755
--- a/scripts/generate_ssl_debug_helpers.py
+++ b/scripts/generate_ssl_debug_helpers.py
@@ -367,7 +367,7 @@
         Generate functions of debug helps
     """
     mbedtls_root = os.path.abspath(
-        mbedtls_root or build_tree.guess_mbedtls_root())
+        mbedtls_root or build_tree.guess_project_root())
     with open(os.path.join(mbedtls_root, 'include/mbedtls/ssl.h')) as f:
         source_code = remove_c_comments(f.read())