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>

# m68k specific configuration file

# depends on EXPERIMENTAL

config ARCH_m68k

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_DEFAULT_BE

    select ARCH_SUPPORT_CPU

     help

      The m68k architecture