refactor(trng): cleanup the existing TRNG support

This patch adds the following changes to complete the existing
TRNG implementation:

1. Adds a feature specific scope for buildlog generation.
2. Updates the docs on the build flag "TRNG_SUPPORT" and its values.
3. Makefile update and improves the existing comments at few sections
for better understanding of the underlying logic.

Change-Id: I3f72f0ccd5c94005a2df87158cf23199d2160d37
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index c90441c..c7b7908 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -16,10 +16,10 @@
 #if ENABLE_RME
 #include <services/rmm_core_manifest.h>
 #endif
+#include <drivers/fwu/fwu_metadata.h>
 #if TRNG_SUPPORT
 #include "plat_trng.h"
-#endif
-#include <drivers/fwu/fwu_metadata.h>
+#endif /* TRNG_SUPPORT */
 #if DRTM_SUPPORT
 #include "plat_drtm.h"
 #endif /* DRTM_SUPPORT */