scripts: use CT_CONFIG_SHELL instead of CT_SHELL

CT_SHELL is undefined.
Thus, the generated wrapper scripts are not executable by the kernel
because they do not contain a valid interpreter.
Use CT_CONFIG_SHELL instead.

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>

reporter_name="Kevin Cernekee"

reporter_url="http://sourceware.org/ml/crossgcc/2011-09/msg00037.html"

reporter_comment="MIPS SDE-compatible toolchain"