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
6 files changed