SPL: Add CONFIG_SPL_ABORT_ON_RAW_IMAGE

When defined, SPL will proceed to another boot method
if the image it has loaded does not have a signature.
This is useful if the subsequent boot methods are much
more complex.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Peng Fan <van.freenix@gmail.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>