Disable U-Boot console if CONFIG_DISABLE_CONSOLE is set

This is a U-Boot part of a fix for "[746-000922] U-Boot: Investigate a way to fully disable the serial debug port."

Add condition which disables a serial console in SPL and U-Boot by activating corresponding routines in following board files:

    seco_mx6
    trizeps8
    trizeps8mini
    trizeps8nano
    trizeps8plus

Herewith there are only three defconfigs, as they should cover the whole spectre of iMX6 and iMX8 boards which are supported in Yocto HAM:

    seco-mx6_no_console_defconfig
    tanaro_v1r1_1gb_no_console_defconfig
    trizeps8plus_no_console_defconfig

A second MR, which adds handling of these defconfigs in Yocto, will follow soon.