MOX: Enable earlyprintk for easier UART debugging
Similar as for Turris Omnia (#347 (closed)), we need similar stuff for Turris MOX:
CONFIG_CMDLINE="earlycon=ar3700_uart,0xd0012000 console=ttyMV0,115200"
CONFIG_CMDLINE_FROM_BOOTLOADER=y
CONFIG_SERIAL_EARLYCON=y
CONFIG_SERIAL_MVEBU_UART=y
CONFIG_SERIAL_MVEBU_CONSOLE=y