kernel/linux: use a custom kernel source directory

Allows using either a tarball or a directory as the custom kernel
source location.

Signed-off-by: Vincent BENOIT <sinseman44@gmail.com>
[yann.morin.1998@anciens.enib.fr: fix space damage, detailed commit message]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.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 $?