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-options.rst b/docs/getting_started/build-options.rst
index a891380..8e62380 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -107,6 +107,9 @@
 -  ``V``: Verbose build. If assigned anything other than 0, the build commands
    are printed. Default is 0.
 
+-  ``ENABLE_REALM_PAYLOAD_TESTS``: This option builds and packs Realm payload tests for
+   RME enabled stack. Default is 0.
+
 Arm FVP Platform Specific Build Options
 ---------------------------------------
 
@@ -152,7 +155,7 @@
 -  ``TFTF_MAX_IMAGE_SIZE``: The option needs to be either set by the user or
    by the platform makefile to specify the maximum size of TFTF binary. This
    is needed so that the Realm payload binary can be appended to TFTF binary
-   via ``make pack_realm`` build command.
+   via ``make ENABLE_REALM_PAYLOAD_TESTS=1 tftf`` build command.
 
 FWU-specific Build Options
 --------------------------