pygrub: fix extlinux parsing

pygrub: fix extlinux parsing

pygrub was unable to parse extlinux config files correctly, exactly
the ones like:

LABEL grsec
  KERNEL vmlinuz-3.0.10-grsec
  APPEND initrd=initramfs-3.0.10-grsec
root=UUID=cfd4a7b4-8c40-4025-b877-8205f1c622ee
modules=sd-mod,usb-storage,ext4 xen quiet

This patch fixes it, adding a new case when parsing the "append" line,
that searches for the initrd image.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Campbell <ian.campbell.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>