fwu: mtd: Fix dfu_alt_info generation for 2 images per bank

Code rewrites the last char of size with adding &. It is visible from
dfu_alt_info print before this patch:

Make dfu_alt_info: 'mtd nor0=bank0 raw 2320000 80000;bank1 raw 27a0000
8000&mtd nor0=bank0 raw 23a0000 4000000;bank1 raw 2820000 4000000'

And after it:
Make dfu_alt_info: 'mtd nor0=bank0 raw 2320000 80000;bank1 raw 27a0000
80000&mtd nor0=bank0 raw 23a0000 4000000;bank1 raw 2820000 4000000'

Size for bank0 and bank1 must be the same because it is the same image.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
diff --git a/lib/fwu_updates/fwu_mtd.c b/lib/fwu_updates/fwu_mtd.c
index b73111a..69cd3d7 100644
--- a/lib/fwu_updates/fwu_mtd.c
+++ b/lib/fwu_updates/fwu_mtd.c
@@ -175,8 +175,10 @@
 
 		l = strlen(buf);
 		/* Replace the last ';' with '&' if there is another image. */
-		if (i != CONFIG_FWU_NUM_IMAGES_PER_BANK - 1 && l)
-			buf[l - 1] = '&';
+		if (i != CONFIG_FWU_NUM_IMAGES_PER_BANK - 1 && l) {
+			buf[l] = '&';
+			buf++;
+		}
 		len -= l;
 		buf += l;
 	}