arch: Refactor BR2_SOFT_FLOAT into per-architecture options

As we are going to introduced a more advanced support of floating
point options for the ARM architecture, we need to adjust how the
soft-float option is handled. We replace the current hidden option
BR2_PREFER_SOFT_FLOAT option and the visible BR2_SOFT_FLOAT option by:

 * A global hidden BR2_SOFT_FLOAT option, defined in arch/Config.in,
   that tells whether the architecture-specific code is using software
   emulated floating point. This hidden option can be used throughout
   Buildroot to determine whether soft float is used or not.

 * Per-architecture visible BR2_<arch>_SOFT_FLOAT options, for the
   architecture for which it makes sense, which allows users to select
   soft float emulation when needed.

This change will allow each architecture to have a different way of
presenting its floating point capabilities.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>