samples: use savedefconfig when saving samples

When saving a sample, use savedefconfig instead of copying
the full .config file.

This reduces the saved .config, and reduces clutter when it
is later upgraded.

Also use defconfig when retrieving a sample.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 #!/bin/sh

 call_to=@@DEFAULT_LD@@

 case "${CTNG_LD_IS}" in

     bfd)    call_to=bfd;;

     gold)   call_to=gold;;

 esac

 exec "${0}.${call_to}" "$@"

 exit $?