boot loader: support root=UUID

As mentioned when introducing the VM images
(https://bugzilla.yoctoproject.org/show_bug.cgi?id=7374), the
resulting images only work when the image is mounted as a disk that
results in the hard-coded path (/dev/sda in the current
default). Using the file system UUID to find the rootfs is more
flexible.

To enable this for boot-direct.bbclass and thus image-vm.bbclass (aka
FSTYPEs vdi/vmdk/qcow2), set SYSLINUX_ROOT =
"root=UUID=<<uuid-of-rootfs>>". The rootfs image must use an ext file
system.

The special string will get replaced in the APPEND line with the
actual UUID when the boot loader (grub-efi, syslinux or gummiboot)
writes the boot loader configuration files. At that time, the rootfs
image has already been created and its UUID can be extracted using
"tune2fs -l", which also should be available because the
e2fsprogs-native tools were needed to create the image in the first
place.

(From OE-Core rev: 1e29d77d0d33ee216b43022439876863f0db39bb)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/syslinux.bbclass

@@ -34,6 +34,9 @@ ISO_BOOTCAT = "isolinux/boot.cat"
 MKISOFS_OPTIONS = "-no-emul-boot -boot-load-size 4 -boot-info-table"
 APPEND_prepend = " ${SYSLINUX_ROOT} "
 
+# Need UUID utility code.
+inherit fs-uuid
+
 syslinux_populate() {
 	DEST=$1
 	BOOTDIR=$2
@@ -177,7 +180,7 @@ python build_syslinux_cfg () {
                     cfgfile.write('initrd=/initrd ')
 
                 cfgfile.write('LABEL=%s '% (label))
-
+                append = replace_rootfs_uuid(d, append)
                 cfgfile.write('%s %s\n' % (append, btype[1]))
             else:
                 cfgfile.write('APPEND %s\n' % btype[1])