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

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi