iso element to use 'search --label' for grub

Current iso element uses 'search --file' to find the
grub root.  This is a wrong approach as it may find
some other partition containing /vmlinuz. Instead modify
it to search for grub root by the label of ISO image.

Closes-Bug: 1384826
Change-Id: Id4217be3420597bed2f80a96788928259ec91582

elements/iso/cleanup.d/100-build-iso

@@ -20,7 +20,7 @@ set -o pipefail
 function build_efiboot_img() {
 
     cat > "$TMP_BUILD_DIR/grub-embedded.cfg" << END_CONFIG
-search --file --set=root /vmlinuz
+search --set=root --label VMEDIA_BOOT_ISO
 set prefix=(\$root)/EFI/BOOT/
 END_CONFIG
 
@@ -127,7 +127,7 @@ END_CONFIG
 
     # Create the ISO
     echo "Generating the ISO"
-    $MKISOFS -r -V "INSTALL_IMAGE" -cache-inodes -J -l \
+    $MKISOFS -r -V "VMEDIA_BOOT_ISO" -cache-inodes -J -l \
     -b isolinux/isolinux.bin  -no-emul-boot \
     -boot-load-size 4 -boot-info-table \
     -eltorito-alt-boot -e isolinux/efiboot.img \