kernel/linux: fix using custom tarball

The custom-tarball symlink was created in CT_SRC_DIR, when it
should be created in CT_TARBALLS_DIR.

Reported-by: Guylhem Aznar <crossgcc@guylhem.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Bart vdr. Meulen <bartvdrmeulen@gmail.com>"

reporter_url=""

reporter_comment="Config to build cross-compiler with mingw as kernel type"