feat(sptool): generate `ARM_BL2_SP_LIST_DTS` file from `sp_layout.json`

TF-A makefile accepts a device-tree snippet to override hardcoded SP
nodes, via the `ARM_BL2_SP_LIST_DTS` variable. However the SPs declared
in `ARM_BL2_SP_LIST_DTS` must be in the same order as they are in the
FIP image, otherwise hash authentication will fail when loaded by BL2.

This patch generates the `ARM_BL2_SP_LIST_DTS` file from the
`sp_layout.json` file. The SPs in the FIP image are also generated from
`sp_layout.json`, so this ensures that there is only one source of truth
for the SP list, removing the possibility to have the lists disagree
with each other.

Signed-off-by: Karl Meakin <karl.meakin@arm.com>
Change-Id: I7d76715135c596605c6a02aad5196d967dfeb1ce
diff --git a/Makefile b/Makefile
index 4b96be1..46b712e 100644
--- a/Makefile
+++ b/Makefile
@@ -139,6 +139,7 @@
 SPTOOLPATH		?=	tools/sptool
 SPTOOL			?=	${SPTOOLPATH}/sptool.py
 SP_MK_GEN		?=	${SPTOOLPATH}/sp_mk_generator.py
+SP_DTS_LIST_FRAGMENT	?=	${BUILD_PLAT}/sp_list_fragment.dts
 
 # Variables for use with ROMLIB
 ROMLIBPATH		?=	lib/romlib
@@ -1542,8 +1543,8 @@
 
 # Add Secure Partition packages
 ifeq (${NEED_SP_PKG},yes)
-$(BUILD_PLAT)/sp_gen.mk: ${SP_MK_GEN} ${SP_LAYOUT_FILE} | ${BUILD_PLAT}
-	${Q}${PYTHON} "$<" "$@" $(filter-out $<,$^) $(BUILD_PLAT) ${COT}
+$(BUILD_PLAT)/sp_gen.mk : ${SP_MK_GEN} ${SP_LAYOUT_FILE} | ${BUILD_PLAT}
+	${PYTHON} "$<" "$@" $(filter-out $<,$^) $(BUILD_PLAT) ${COT} ${SP_DTS_LIST_FRAGMENT}
 sp: $(DTBS) $(BUILD_PLAT)/sp_gen.mk $(SP_PKGS)
 	@${ECHO_BLANK_LINE}
 	@echo "Built SP Images successfully"