Add sfdisk to the image

Use sfdisk to format SD cards or USB sticks.

Signed-off-by: Stefano Babic <sbabic@denx.de>

recipes-extended/images/swupdate-image.bb

@@ -16,6 +16,7 @@ IMAGE_INSTALL = "base-files \
 		swupdate \
 		swupdate-www \
 		sysvinit \
+		util-linux-sfdisk \
 		initscripts \
 		 "