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/services/std_svc/std_svc_setup.c b/services/std_svc/std_svc_setup.c
index 08d16e2..2884a3b 100644
--- a/services/std_svc/std_svc_setup.c
+++ b/services/std_svc/std_svc_setup.c
@@ -74,7 +74,10 @@
 	sdei_init();
 #endif
 
+#if TRNG_SUPPORT
+	/* TRNG initialisation */
 	trng_setup();
+#endif /* TRNG_SUPPORT */
 
 #if DRTM_SUPPORT
 	if (drtm_setup() != 0) {
@@ -172,7 +175,8 @@
 		return trng_smc_handler(smc_fid, x1, x2, x3, x4, cookie, handle,
 				flags);
 	}
-#endif
+#endif /* TRNG_SUPPORT */
+
 #if ENABLE_RME
 
 	if (is_rmmd_el3_fid(smc_fid)) {