Make the changes easier to backport

The code replaced in this patch was not compatible with the
development_2.x branch.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/tests/scripts/test_psa_compliance.py b/tests/scripts/test_psa_compliance.py
index 07fa76e..d6fe8c4 100755
--- a/tests/scripts/test_psa_compliance.py
+++ b/tests/scripts/test_psa_compliance.py
@@ -16,9 +16,8 @@
 def main():
     mbedtls_dir = os.getcwd()
 
-    mbedcrypto_lib = 'library/libmbedcrypto.a'
-    if not os.path.exists(mbedcrypto_lib):
-        subprocess.check_call(['make', mbedcrypto_lib])
+    if not os.path.exists('library/libmbedcrypto.a'):
+        subprocess.check_call(['make', '-C', 'library', 'libmbedcrypto.a'])
 
     psa_arch_tests_dir = 'psa-arch-tests'
     try: