toolchain-external: do not allow the user to select hard/soft float

Commit ba92d6ef made hard float the
default when Cortex-A8 and Cortex-A9. The problem it was trying to fix
is that the newer Linaro toolchains (2012.05 and 2012.06) are
hard-float, so the default selection of soft-float enabled on ARM
doesn't work for those toolchains.

Unfortunately, not selecting soft-float causes problems with
the Crosstool-NG backend at the moment.

As an intermediate solution, make the soft float option disappear when
using external toolchain: the toolchain will decide by itself whether
to generate hard float or soft float code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>