refactor(build): distinguish BL2 as TF-A entry point and BL2 running at EL3

BL2_AT_EL3 is an overloaded macro which has two uses:
	1. When BL2 is entry point into TF-A(no BL1)
	2. When BL2 is running at EL3 exception level
These two scenarios are not exactly same even though first implicitly
means second to be true. To distinguish between these two use cases we
introduce new macros.
BL2_AT_EL3 is renamed to RESET_TO_BL2 to better convey both 1. and 2.
Additional macro BL2_RUNS_AT_EL3 is added to cover all scenarious where
BL2 runs at EL3 (including four world systems).

BREAKING CHANGE: BL2_AT_EL3 renamed to RESET_TO_BL2 across the
repository.

Change-Id: I477e1d0f843b44b799c216670e028fcb3509fb72
Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
diff --git a/docs/design/firmware-design.rst b/docs/design/firmware-design.rst
index f08608c..97f3550 100644
--- a/docs/design/firmware-design.rst
+++ b/docs/design/firmware-design.rst
@@ -25,7 +25,6 @@
 :ref:`Translation (XLAT) Tables Library`.
 
 TF-A can be built to support either AArch64 or AArch32 execution state.
-
 .. note::
 
  The descriptions in this chapter are for the Arm TrustZone architecture.
@@ -484,8 +483,8 @@
 as its only purpose is to ensure TF-A BL2 is entered at S-EL1. To avoid
 this waste, a special mode enables BL2 to execute at EL3, which allows
 a non-TF-A Boot ROM to load and jump directly to BL2. This mode is selected
-when the build flag BL2_AT_EL3 is enabled. The main differences in this
-mode are:
+when the build flag RESET_TO_BL2 is enabled.
+The main differences in this mode are:
 
 #. BL2 includes the reset code and the mailbox mechanism to differentiate
    cold boot and warm boot. It runs at EL3 doing the arch
@@ -2752,7 +2751,7 @@
 
 --------------
 
-*Copyright (c) 2013-2022, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2023, Arm Limited and Contributors. All rights reserved.*
 
 .. _Power State Coordination Interface PDD: http://infocenter.arm.com/help/topic/com.arm.doc.den0022d/Power_State_Coordination_Interface_PDD_v1_1_DEN0022D.pdf
 .. _SMCCC: https://developer.arm.com/docs/den0028/latest