feat(rme) : introduce new build flag for RME stack

* Introduce new build flag ENABLE_REALM_PAYLOAD_TESTS to build
  Realm Payload Tests and pack realm image to tftf.bin.
* Also enable PAuth for TFTF and Realm Payload by default
  for RME stack.
* This commit deprecates the use of `pack_realm` build target.
* The patch also modifies the user guide instructions to build
  and pack realm payload tests.

NOTE: The `pack_realm` build target, even though deprecated,
is still available for compatibility reasons. It will be
removed in a future commit.

Signed-off-by: Shruti Gupta <shruti.gupta@arm.com>
Change-Id: Iaa83651c2f41152a5a2bf4d0b9caa550f302da6b
diff --git a/docs/getting_started/build.rst b/docs/getting_started/build.rst
index 034a99e..70b7edc 100644
--- a/docs/getting_started/build.rst
+++ b/docs/getting_started/build.rst
@@ -111,14 +111,10 @@
 
 ::
 
-    make PLAT=<platform> realm
+    make PLAT=<platform> ENABLE_REALM_PAYLOAD_TESTS=1 tftf
 
-The generated ``realm.bin`` needs to be packaged as part of ``tftf.bin`` to
-be used as a single BL33 image and can be done using the following command:
-
-::
-
-    make PLAT=<platform> pack_realm
+The generated ``realm.bin`` is packaged as part of ``tftf.bin``
+to be used as a single BL33 image.
 
 Please refer to the `TF-A RME documentation`_ for build and run instructions.