TA-dev-kit: build TA with -mfloat-abi=soft

Solves the same problem as in commit e46048ea67f7...

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>