kernel/linux: symlink custom kernel source dir with '-f'

build fails to symlink to custom kernel dir when the build is not the first time
because of 'ln -s' without '-f' option.

Signed-off-by: "Jang, Bongseo" <graycells@gmail.com>
Message-ID: <543e2981f2b723ecd850.1348370892@localhost.localdomain>
PatchWork-ID: 186178

 #!/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 $?