[yocto] Append recipe LICENCE checksum problem

[p_j_r_m]

I have a layer meta-mycompany where i want to append to the grub_2.00 recipe placed at openembedded-core layer just to add my custom grub.cfg file.
Also, I see there is ALREADY a grub-2.00.bbappend file in my BSP layer (meta-montavista-1.4).My layer has highest priority.

So I wrote a recipe called grub-2.00.bbappend like this in my layer:

FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
SRC_URI = "file://grub${MYBOARD}.cfg "
FILES_${PN}_append = "${base_prefix}/boot"

do_install_append() {
  if [ ! -d ${D}${base_prefix}/boot/grub ]
  then
     install -m 0755 -d ${S}${base_prefix}/boot/grub ${D}${base_prefix}/boot/grub
  fi
  install -m 0755 ${WORKDIR}/grub${MYBOARD}.cfg ${D}${base_prefix}/boot/grub/grub.cfg
}

$MYBOARD is a custom name just to pick up the proper grub file for each platform. 

But when i do a "bitbake grub" I get:

NOTE: Executing RunQueue Tasks
ERROR: Function failed: grub: LIC_FILES_CHKSUM points to an invalid file: /opt/work/tmp/work/x86_64-montavista-linux/grub/2.00-r1.2.2/grub-2.00/COPYING
ERROR: Logfile of failure stored in: /opt/work/tmp/work/x86_64-montavista-linux/grub/2.00-r1.2.2/temp/log.do_configure.628
Log data follows:
| DEBUG: Executing python function sysroot_cleansstate
...
| DEBUG: Python function do_qa_configure finished
| ERROR: Function failed: grub: LIC_FILES_CHKSUM points to an invalid file: /opt/work/tmp/work/x86_64-montavista-linux/grub/2.00-r1.2.2/grub-2.00/COPYING
ERROR: Task 5 (/opt/work/tmp/layers/openembedded-core-1.4-1411101352/openembedded-core-1.4-1411101352/meta/recipes-bsp/grub/grub_2.00.bb, do_configure) failed with exit code '1'

If i take my layer out of my conf/extra-layers.conf ,recipe builds ok.

My layer is:

releases
├── meta-mycompany
│   ├── conf
│   │   └── layer.conf
│   ├── README
│   ├── recipes
│       ├── grub
│       │   ├── files
│       │   │   ├── grubatca.cfg
│       │   │   └── grubmni.cfg
│       │   └── grub_2.00.bbappend
│       ├── images
│          ├── mycompany-atca-image.bb
│          └── mycompany-atca-image-buildcf.bb
├── meta-mycompany.tar.bz2
├── meta-mycompany.tar.bz2.md5

Do you know what is the problem? 
Must an .append recipe also provide a LICENCE file? Where?

Thank you