configs/stm32mp157: use common genimage config file for stm32mp157 boards

It will be useful for the upcoming introducing stm32mp157a dk2 board.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/stm32mp157c_dk2_defconfig

@@ -3,7 +3,7 @@ BR2_cortex_a7=y
 BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_4=y
 BR2_ROOTFS_OVERLAY="board/stmicroelectronics/stm32mp157c-dk2/overlay/"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
-BR2_ROOTFS_POST_SCRIPT_ARGS="-c board/stmicroelectronics/stm32mp157c-dk2/genimage.cfg"
+BR2_ROOTFS_POST_SCRIPT_ARGS="-c board/stmicroelectronics/common/stm32mp157/genimage.cfg"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.4.7"