dahdi-tools: unify dependencies comments

The threads dependency comment is currently shown even though the
toolchain supports threads, only because kernel build is disabled.

Merge the kernel and threads comments. This is similar to what we have
in other packages that need the kernel.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>