grub-efi-cfg: enable per-label APPEND override

grub-efi-cfg: enable per-label APPEND override

For legacy bios boot configurations, syslinux supports multiple
labels with per-label APPEND definitions.  grub-efi-cfg supports
multiple labels, but only a single APPEND definition.

Enable optional per-label APPEND definitions for grub EFI, with
variable names prefixed by "grub_" to isolate grub definitions from
syslinux defintions.

Example use from an ISO image recipe that inherits grub-efi-cfg:

 LABELS_LIVE="foo bar"
 APPEND_grub_foo = "linuxcmdline"

No change in behavior for those using APPEND without overrides.

Signed-off-by: Rich Persaud <rp@stacktrust.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>