sdcard_image-rpi: fix absolute path in vfat symlink
sdcard_image-rpi: fix absolute path in vfat symlink

Fix bitbake complaining with:

ERROR: core-image-minimal-1.0-r0 do_image_complete: sstate found an absolute
path symlink [...].vfat pointing at [...].vfat. Please replace this with a
relative link.

This patch redefines SDIMG_VFAT as a file name instead of a full absolute path,
then fixing the symlink creation by pointing to a relative target. The cp
command that deploys the artifact is changed accordingly to include the
destination path.

Signed-off-by: Andrea Galbusera <6af94ec2abb4591715fcd9dabad664a550d900de@gmail.com>

d21d3q/meta-raspberrypi,leon-anavi/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,d21d3q/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,kraj/meta-raspberrypi,leon-anavi/meta-raspberrypi,schnitzeltony/meta-raspberrypi,d21d3q/meta-raspberrypi,d21d3q/meta-raspberrypi,kraj/meta-raspberrypi,leon-anavi/meta-raspberrypi,kraj/meta-raspberrypi,schnitzeltony/meta-raspberrypi