[SPLASH] enable splash screen to U-boot and Linux on SECO i.MX boards

• U-boot: the logo image is loaded from mmc 1:1 using the splashsource library.
• Kernel: the logo image must be installed in the source before compiling, and the way it's displayed is configured through the fbcon kernel command line arg. See seco_environment.h.
• Tested on a62 and c12.