Invoke config.py instead of config.pl

git grep -Fl /config.pl | xargs sed -i -e 's!/config\.pl!/config.py!g'

Also:
* Change one comment in include/mbedtls/check_config.h.
* Change PERL to PYTHON in CMakeLists.txt.
diff --git a/scripts/footprint.sh b/scripts/footprint.sh
index 697972f..6cabcb9 100755
--- a/scripts/footprint.sh
+++ b/scripts/footprint.sh
@@ -62,9 +62,9 @@
     fi
 
     {
-        scripts/config.pl unset MBEDTLS_TIMING_C || true
-        scripts/config.pl unset MBEDTLS_FS_IO || true
-        scripts/config.pl --force set MBEDTLS_NO_PLATFORM_ENTROPY || true
+        scripts/config.py unset MBEDTLS_TIMING_C || true
+        scripts/config.py unset MBEDTLS_FS_IO || true
+        scripts/config.py --force set MBEDTLS_NO_PLATFORM_ENTROPY || true
     } >/dev/null 2>&1
 
     make clean >/dev/null