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/gummiboot.bbclass

@@ -15,6 +15,9 @@ GUMMIBOOT_CFG ?= "${S}/loader.conf"
 GUMMIBOOT_ENTRIES ?= ""
 GUMMIBOOT_TIMEOUT ?= "10"
 
+# Need UUID utility code.
+inherit fs-uuid
+
 efi_populate() {
         DEST=$1
 
@@ -108,6 +111,7 @@ python build_efi_cfg() {
             lb = "install-efi"
         entrycfg.write('options LABEL=%s ' % lb)
         if append:
+            append = replace_rootfs_uuid(d, append)
             entrycfg.write('%s' % append)
         entrycfg.write('\n')
         entrycfg.close()