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

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_WITH_CPU

 ##

 ## help The m68k architecture