config/toolchain: force use of sysroot if OBSOLETE is not set

Use of the sysroot is highly recommended, and the non-sysroot case is
both obsolete and not widely tested.

Before the non-sysroot case can go away, deprecate it.

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"