toolchain-external wrapper: don't pass march/mcpu if mtune is on cmdline

Before commit 5715d2dc, the external
toolchain wrapper would not pass its own march/mcpu/mtune flags to the real
compiler if at least one of them was passed on the wrapper command-line.

The mentioned commit intended to remove the passing of an mtune parameter
coming from Buildroot, which was always empty after some other refactoring,
but the changes have the side-effect that march/mcpu is now also passed when
mtune is already given on the command-line. In that case, only mtune should
be passed to the real compiler.

Restore part of the original toolchain wrapper code to check the presence of
mtune on the command-line.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>