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>

board/stmicroelectronics/stm32mp157c-dk2/genimage.cfg

@@ -1,23 +0,0 @@
-image sdcard.img {
-	hdimage {
-		gpt = "true"
-	}
-
-	partition fsbl1 {
-		image = "u-boot-spl.stm32"
-	}
-
-	partition fsbl2 {
-		image = "u-boot-spl.stm32"
-	}
-
-	partition ssbl {
-		image = "u-boot.img"
-	}
-
-	partition rootfs {
-		image = "rootfs.ext4"
-		partition-type = 0x83
-		bootable = "yes"
-	}
-}