Add MBEDTLS_ENTROPY_HARDWARE_ALT

Makes it easier for an external module to plug its hardware entropy collector.

library/entropy.c

diff --git a/library/entropy.c b/library/entropy.c
index fa3dcde..4dddb75 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -86,6 +86,10 @@
     mbedtls_entropy_add_source( ctx, mbedtls_havege_poll, &ctx->havege_data,
                         MBEDTLS_ENTROPY_MIN_HAVEGE );
 #endif
+#if defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+    mbedtls_entropy_add_source( ctx, mbedtls_hardware_poll, NULL
+                                MBEDTLS_ENTROPY_MIN_HARDWARE );
+#endif
 #endif /* MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES */
 }