configs: stm32m15: support STM32MP_BOARD_EXTRA_ENV for st boards

Correctly handle STM32MP_BOARD_EXTRA_ENV define in stm32mp15_st_common.h;
the STM32MP_BOARD_EXTRA_ENV is added in CONFIG_EXTRA_ENV_SETTINGS
definition, as it is done "stm32mp15_st_common.h"

Without this patch, the content of STM32MP_BOARD_EXTRA_ENV is not used in
the default environment for STMicroelectronics boards.

Fixes: 806c4dd31532 ("configs: stm32mp1: set the console variable for extlinux.conf")
Reported-by: Gatien CHEVALLIER <gatien.chevallier@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/include/configs/stm32mp15_st_common.h b/include/configs/stm32mp15_st_common.h
index 37b216e..6bdc286 100644
--- a/include/configs/stm32mp15_st_common.h
+++ b/include/configs/stm32mp15_st_common.h
@@ -48,7 +48,8 @@
 	ST_STM32MP1_BOOTCMD \
 	STM32MP_PARTS_DEFAULT \
 	BOOTENV \
-	STM32MP_EXTRA
+	STM32MP_EXTRA \
+	STM32MP_BOARD_EXTRA_ENV
 
 #endif
 #endif