toolchain/external: Arago armv7 toolchain really requires a VFPv3

The Arago armv7 toolchain really requires a VFPv3 unit, so only expose
it to the user when the CPU actually has such a VFP unit

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>