tools: mkimage: fix imximage header size

imximage header size is 4-byte, not 8-byte aligned.
This produces .imx images that a Vybrid cannot boot
on.

Fix by adding a "padding" field in header.

Signed-off-by: Albert ARIBAUD (3ADEV) <albert.aribaud@3adev.fr>
diff --git a/tools/imximage.h b/tools/imximage.h
index 36fe095..a913329 100644
--- a/tools/imximage.h
+++ b/tools/imximage.h
@@ -129,6 +129,7 @@
 	ivt_header_t header;
 	write_dcd_command_t write_dcd_command;
 	dcd_addr_data_t addr_data[MAX_HW_CFG_SIZE_V2];
+	uint32_t padding[1]; /* end up on an 8-byte boundary */
 } dcd_v2_t;
 
 typedef struct {