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/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 65ceb7f..772b28d 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -270,7 +270,7 @@
 # Software Delegated Exception support
 SDEI_SUPPORT			:= 0
 
-# True Random Number firmware Interface
+# True Random Number firmware Interface support
 TRNG_SUPPORT			:= 0
 
 # SMCCC PCI support