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/Makefile b/Makefile
index a14d4d8..6d836e8 100644
--- a/Makefile
+++ b/Makefile
@@ -1079,6 +1079,7 @@
         ENABLE_MPMM_FCONF \
         SIMICS_BUILD \
         FEATURE_DETECTION \
+	TRNG_SUPPORT \
 )))
 
 $(eval $(call assert_numerics,\