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/make_helpers/defaults.mk b/make_helpers/defaults.mk
index a66123a..4f94cd0 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016-2022, Arm Limited. All rights reserved.
+# Copyright (c) 2016-2023, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -30,13 +30,13 @@
 BASE_COMMIT			:= origin/master
 
 # Execute BL2 at EL3
-BL2_AT_EL3			:= 0
+RESET_TO_BL2			:= 0
 
 # Only use SP packages if SP layout JSON is defined
 BL2_ENABLE_SP_LOAD		:= 0
 
 # BL2 image is stored in XIP memory, for now, this option is only supported
-# when BL2_AT_EL3 is 1.
+# when RESET_TO_BL2 is 1.
 BL2_IN_XIP_MEM			:= 0
 
 # Do dcache invalidate upon BL2 entry at EL3