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/fs-uuid.bbclass

@@ -0,0 +1,24 @@
+# Extract UUID from ${ROOTFS}, which must have been built
+# by the time that this function gets called. Only works
+# on ext file systems and depends on tune2fs.
+def get_rootfs_uuid(d):
+    import subprocess
+    rootfs = d.getVar('ROOTFS', True)
+    output = subprocess.check_output(['tune2fs', '-l', rootfs])
+    for line in output.split('\n'):
+        if line.startswith('Filesystem UUID:'):
+            uuid = line.split()[-1]
+            bb.note('UUID of %s: %s' % (rootfs, uuid))
+            return uuid
+    bb.fatal('Could not determine filesystem UUID of %s' % rootfs)
+
+# Replace the special <<uuid-of-rootfs>> inside a string (like the
+# root= APPEND string in a syslinux.cfg or gummiboot entry) with the
+# actual UUID of the rootfs. Does nothing if the special string
+# is not used.
+def replace_rootfs_uuid(d, string):
+    UUID_PLACEHOLDER = '<<uuid-of-rootfs>>'
+    if UUID_PLACEHOLDER in string:
+        uuid = get_rootfs_uuid(d)
+        string = string.replace(UUID_PLACEHOLDER, uuid)
+    return string

meta/classes/grub-efi.bbclass

@@ -26,6 +26,9 @@ GRUB_OPTS ?= "serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1"
 
 EFIDIR = "/EFI/BOOT"
 
+# Need UUID utility code.
+inherit fs-uuid
+
 efi_populate() {
 	# DEST must be the root of the image so that EFIDIR is not
 	# nested under a top level directory.
@@ -129,6 +132,7 @@ python build_efi_cfg() {
             initrd = localdata.getVar('INITRD', True)
 
             if append:
+                append = replace_rootfs_uuid(d, append)
                 cfgfile.write('%s' % (append))
             cfgfile.write(' %s' % btype[1])
             cfgfile.write('\n')

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()

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])