build: clarify linker script generation

The following build system variables have been renamed:

- `LINKERFILE` -> `DEFAULT_LINKER_SCRIPT`
- `BL_LINKERFILE` -> `DEFAULT_LINKER_SCRIPT_SOURCE`
- `<IMAGE>_LINKERFILE` -> `<IMAGE>_DEFAULT_LINKER_SCRIPT_SOURCE`

These new names better reflect how each variable is used:

1. the default linker script is passed via `-dT` instead of `-T`
2. linker script source files are first preprocessed

Additionally, linker scripts are now placed in the build directory
relative to where they exist in the source directory. For example,
the `bl32/sp_min/sp_min.ld.S` would now preprocess to
`sp_min/sp_min.ld` instead of just `bl32.ld`

BREAKING-CHANGE: The `LINKERFILE`, `BL_LINKERFILE` and
 `<IMAGE_LINKERFILE>` build system variables have been renamed. See the
 commit message for more information.

Change-Id: If8cef65dcb8820e8993736702c8741e97a66e6cc
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/bl2/bl2.mk b/bl2/bl2.mk
index 7a973e5..a18abab 100644
--- a/bl2/bl2.mk
+++ b/bl2/bl2.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2013-2021, Arm Limited and Contributors. All rights reserved.
+# Copyright (c) 2013-2023, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -23,12 +23,12 @@
 				bl2/${ARCH}/bl2_el3_exceptions.S	\
 				bl2/${ARCH}/bl2_run_next_image.S	\
 				${GPT_LIB_SRCS}
-BL2_LINKERFILE		:=	bl2/bl2.ld.S
+BL2_DEFAULT_LINKER_SCRIPT_SOURCE := bl2/bl2.ld.S
 
 else ifeq (${BL2_AT_EL3},0)
 # Normal operation, no RME, no BL2 at EL3
 BL2_SOURCES		+=	bl2/${ARCH}/bl2_entrypoint.S
-BL2_LINKERFILE		:=	bl2/bl2.ld.S
+BL2_DEFAULT_LINKER_SCRIPT_SOURCE := bl2/bl2.ld.S
 
 else
 # BL2 at EL3, no RME
@@ -46,5 +46,5 @@
 BL2_SOURCES		+=	lib/cpus/aarch64/dsu_helpers.S
 endif
 
-BL2_LINKERFILE		:=	bl2/bl2_el3.ld.S
+BL2_DEFAULT_LINKER_SCRIPT_SOURCE := bl2/bl2_el3.ld.S
 endif