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: 806c4dd3 ("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>