plat-vexpress: build with -mfloat-abi=soft

OP-TEE on plat-vexpress does not support Neon. This patch prevents generating
Neon instructions when the cross-compiler is configured to use
'-mfloat-abi=hard' by default (such as arm-linux-gnueabihf-gcc).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
various minor cleanups

- Remove unused assembly routines "secure_mmu_xxx".
- arm32/plat-stm:
  . move platform stm definitions of CFG_TEE_xxx configs from conf.mk to
    system_config.in
  . emit error in case some configuration variables are not defined
  .Remove deprecated macro _USE_SLAPORT_LIB
- Comment message format

Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org>