image_types.bbclass: Restore compatibility with previous UBI filesystems

Support for multiple ubi images has broken dozens of machine deployment scripts
in two ways:
Previously, ubi filesystems would be named $IMAGE_NAME as one would expect. The
current version would append "_rootfs" to that name for no reason. Fix it so that
the name for ubi images remains unchanged if there is only one image to build.
Machines would append to IMAGE_CMD_ubi, adding extra image processing of their
own. This is broken now that IMAGE_CMD_ubi became a variable instead of a function.
Make IMAGE_CMD_ubi a function again, this also makes for more logical quotes (I
was surprised to find that " within " would even work).

(From OE-Core rev: b6e64de541b37bc5c558c4ad362a0467291a5609)

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image_types.bbclass

@@ -120,7 +120,11 @@ UBI_VOLNAME ?= "${MACHINE}-rootfs"
 multiubi_mkfs() {
 	local mkubifs_args="$1"
 	local ubinize_args="$2"
-	local vname="_$3"
+	if [ -z "$3" ]; then
+		local vname=""
+	else
+		local vname="_$3"
+	fi
 
 	echo \[ubifs\] > ubinize${vname}.cfg
 	echo mode=ubi >> ubinize${vname}.cfg
@@ -158,7 +162,9 @@ IMAGE_CMD_multiubi () {
 	done
 }
 
-IMAGE_CMD_ubi = "multiubi_mkfs "${MKUBIFS_ARGS}" "${UBINIZE_ARGS}" "${UBI_VOLNAME}""
+IMAGE_CMD_ubi () {
+	multiubi_mkfs "${MKUBIFS_ARGS}" "${UBINIZE_ARGS}"
+}
 
 IMAGE_CMD_ubifs = "mkfs.ubifs -r ${IMAGE_ROOTFS} -o ${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.ubifs ${MKUBIFS_ARGS}"