Omit RSA key generation test if no strong entropy is present

The RSA key generation test needs strong entropy to succeed. This commit captures the presence of a strong entropy
source in a preprocessor flag and only runs the key generation test if that flag is set.

library/entropy.c

diff --git a/library/entropy.c b/library/entropy.c
index d4d1b27..4de1682 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -83,6 +83,9 @@
     mbedtls_havege_init( &ctx->havege_data );
 #endif
 
+    /* Reminder: Update MBEDTLS_ENTROPY_HAVE_STRONG when
+     *           adding more strong entropy sources here. */
+
 #if defined(MBEDTLS_TEST_NULL_ENTROPY)
     mbedtls_entropy_add_source( ctx, mbedtls_null_entropy_poll, NULL,
                                 1, MBEDTLS_ENTROPY_SOURCE_STRONG );