Adds a check and warning for the null entropy option

If the option MBEDTLS_TEST_NULL_ENTROPY is enabled, the cmake generated
makefile will generate an error unless a UNSAFE_BUILD switch is also enabled.

Equally, a similar warning will always be generated if the Makefile is built,
and another warning is generated on every compilation of entropy.c.

This is to ensure the user is aware of what they're doing when they enable the
null entropy option.

Makefile

@@ -6,7 +6,7 @@ PREFIX=mbedtls_
 
 .PHONY: all no_test programs lib tests install uninstall clean test check covtest lcov apidoc apidoc_clean
 
-all: programs tests
+all: programs tests post_build
 
 no_test: programs
 
@@ -53,6 +53,20 @@ uninstall:
 	done
 endif
 
+WARNING_BORDER      =*******************************************************\n
+NULL_ENTROPY_WARN_L1=****  WARNING!  MBEDTLS_TEST_NULL_ENTROPY defined! ****\n
+NULL_ENTROPY_WARN_L2=****  THIS BUILD HAS NO DEFINED ENTROPY SOURCES    ****\n
+NULL_ENTROPY_WARN_L3=****  AND IS *NOT* SUITABLE FOR PRODUCTION USE     ****\n
+
+NULL_ENTROPY_WARNING=\n$(WARNING_BORDER)$(NULL_ENTROPY_WARN_L1)$(NULL_ENTROPY_WARN_L2)$(NULL_ENTROPY_WARN_L3)$(WARNING_BORDER)
+
+# Post build steps
+post_build:
+	# If NULL Entropy is configured, display an appropriate warning
+	-scripts/config.pl get MBEDTLS_TEST_NULL_ENTROPY && ([ $$? -eq 0 ]) && \
+	    echo '$(NULL_ENTROPY_WARNING)'
+
+
 clean:
 	$(MAKE) -C library clean
 	$(MAKE) -C programs clean