[RPI][SPLASH] patch u-boot to display splash bmp from sd

- display the splash screen
- enable the splaspos environment variable to configure positioning of the splash image
- enable the splashsource library, which allows to load the logo image from a custom source (see README.splashprepare)
- quiet console messages (see README.silent), which would otherwise overlap with the logo image
- enable CONFIG_SILENT_U_BOOT_ONLY to prevent U-boot from adding "quiet" to the bootargs, which would also hide the kernel logo
- NOTE: in the future we might want to direct the console output to the GPIO serial instead of silencing it entirely