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/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 6996c17..668af57 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -2468,7 +2468,7 @@
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 This value must be defined to the UUID of the TRNG backend that is specific to
-the hardware after ``plat_trng_setup`` function is called. This value must
+the hardware after ``plat_entropy_setup`` function is called. This value must
 conform to the SMCCC calling convention; The most significant 32 bits of the
 UUID must not equal ``0xffffffff`` or the signed integer ``-1`` as this value in
 w0 indicates failure to get a TRNG source.