u-boot: add support for full image socfpga mkpimage

u-boot: add support for full image socfpga mkpimage

On older SoCFPGA SoCs, there was not enough SRAM to fit the entire
U-Boot, so a SPL model was used. The SPL was therefore the binary that
had to be "prepared" to be loaded by the ROM code using the mkpimage
tool.

With newer SoCFPGA SoCs, there is enough SRAM to fit the entire U-Boot,
and therefore the ROM code directly loads U-Boot. In this case, it's the
real U-Boot image (not the SPL) that needs to be prepared using
mkpimage.

This code adds support for the newer SoCFPGA SoCs, by adjusting the
mkpimage related logic to apply on the SPL if an SPL is enabled, or on
the full U-Boot image otherwise.

Signed-off-by: Lionel Flandrin <lionel@svkt.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>