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/Makefile b/Makefile
index e898975..4f4e437 100644
--- a/Makefile
+++ b/Makefile
@@ -80,11 +80,11 @@
 ifndef WINDOWS
 
 	# If 128-bit keys are configured for CTR_DRBG, display an appropriate warning
-	-scripts/config.pl get MBEDTLS_CTR_DRBG_USE_128_BIT_KEY && ([ $$? -eq 0 ]) && \
+	-scripts/config.py get MBEDTLS_CTR_DRBG_USE_128_BIT_KEY && ([ $$? -eq 0 ]) && \
 	    echo '$(CTR_DRBG_128_BIT_KEY_WARNING)'
 
 	# If NULL Entropy is configured, display an appropriate warning
-	-scripts/config.pl get MBEDTLS_TEST_NULL_ENTROPY && ([ $$? -eq 0 ]) && \
+	-scripts/config.py get MBEDTLS_TEST_NULL_ENTROPY && ([ $$? -eq 0 ]) && \
 	    echo '$(NULL_ENTROPY_WARNING)'
 endif