tools/mkimage: add support for STM32 image format

STM32MP157 bootrom needs a specific header for first boot stage.
This patch adds support of this header in mkimage.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/tools/Makefile b/tools/Makefile
index f38f68e..55efb74 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -103,6 +103,7 @@
 			pblimage.o \
 			pbl_crc32.o \
 			vybridimage.o \
+			stm32image.o \
 			$(ROCKCHIP_OBS) \
 			socfpgaimage.o \
 			lib/sha1.o \