stm32mp1: generate extlinux.conf at build time

Generate extlinux config file, used by U-Boot to driver default boot
sequences, a from template where only the Linux DTB file name is set
from target platform specific name.

Acked-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
diff --git a/stm32mp1.mk b/stm32mp1.mk
index 03fecc7..3709c76 100644
--- a/stm32mp1.mk
+++ b/stm32mp1.mk
@@ -225,11 +225,11 @@
 BR2_PACKAGE_BUSYBOX_WATCHDOG=y
 
 BREXT_BOARD_PATH=$(ROOT)/build/br-ext/board/stmicroelectronics/stm32mp1-tz
-BREXT_BOOTFS_OVERLAY=$(BREXT_BOARD_PATH)/overlay-$(BREXT_FLAVOR)
 BREXT_GENIMAGE_CONFIG=$(BREXT_BOARD_PATH)/genimage.cfg
+BREXT_BOOTFS_OVERLAY=$(BREXT_BOARD_PATH)/overlay
 
 BR2_PACKAGE_HOST_GENIMAGE=y
-BR2_ROOTFS_POST_SCRIPT_ARGS="$(BREXT_GENIMAGE_CONFIG) $(BINARIES_PATH) $(BREXT_BOOTFS_OVERLAY) $(STM32MP1_DTS_BASENAME)"
+BR2_ROOTFS_POST_SCRIPT_ARGS="$(BREXT_GENIMAGE_CONFIG) $(BINARIES_PATH) $(BREXT_BOOTFS_OVERLAY) $(STM32MP1_DTS_BASENAME) $(STM32MP1_DTS_LINUX)"
 BR2_ROOTFS_POST_IMAGE_SCRIPT=$(BREXT_BOARD_PATH)/post-image.sh
 
 ifeq ($(WITH_RPMB_TEST),y)