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>

tools/imximage.h

@@ -129,6 +129,7 @@ typedef struct {
 	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 {