toolchain: remove the old BR2_VFP_FLOAT option

Now that we have a much better way of selecting between the various
VFP versions and capabilities, the BR2_VFP_FLOAT version no longer
makes sense. This commit gets rid of it, and adds the appropriate
Config.in.legacy code.

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