Provide user option to skip SPL signature verification for cases where
u-boot is build with SPL support but full U-Boot is also verified
without SPL.

If you want to support this feature please add env__spl_skipped = True
to your boardenv configuration file.

For example Xilinx Zynq is using this feature where the same U-Boot
binary is checked with SPL and without SPL(with FSBL).

Signed-off-by: Michal Simek <michal.simek@xilinx.com>