SPL: Add signature verification when loading image

U-boot proper signature is not verified by SPL on most platforms
even config SPL_FIT_SIGNATURE is enabled. Only fsl-layerscape
platform support secure boot in platform specific code. So
verified boot cannot be achieved if u-boot proper is loaded by
SPL.

This patch add signature verification to u-boot proper images
when loading FIT image in SPL. It is tested on Allwinner bananapi
zero board with H2+ SoC.

Signed-off-by: Jun Nie <jun.nie@linaro.org>