feat(plat/st): add STM32MP_EMMC_BOOT option

Added a new STM32MP_EMMC_BOOT option, which is used to look for SSBL in
the same eMMC boot partition TF-A booted from at a fixed 256k offset. In
case STM32 image header is not found, the boot process rolls back to a
GPT partition look-up scheme.

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Change-Id: I85a87dc9ae7f2b915ed8e584be80f4b3588efc48
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index 50fb1b7..128dbc4 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -42,6 +42,7 @@
 STM32MP_RAW_NAND	?=	0
 STM32MP_SPI_NAND	?=	0
 STM32MP_SPI_NOR		?=	0
+STM32MP_EMMC_BOOT	?=	0
 
 ifeq ($(filter 1,${STM32MP_EMMC} ${STM32MP_SDMMC} ${STM32MP_RAW_NAND} \
 	${STM32MP_SPI_NAND} ${STM32MP_SPI_NOR}),)
@@ -77,6 +78,7 @@
 		STM32MP_RAW_NAND \
 		STM32MP_SPI_NAND \
 		STM32MP_SPI_NOR \
+		STM32MP_EMMC_BOOT \
 		PLAT_XLAT_TABLES_DYNAMIC \
 )))
 
@@ -93,6 +95,7 @@
 		STM32MP_RAW_NAND \
 		STM32MP_SPI_NAND \
 		STM32MP_SPI_NOR \
+		STM32MP_EMMC_BOOT \
 		PLAT_XLAT_TABLES_DYNAMIC \
 		STM32_TF_A_COPIES \
 		PLAT_PARTITION_MAX_ENTRIES \