Disabled by default.

This keeps dependency on mdef, sysgen and other stuff which complicates
build integration.

To enable options which may be incompatible with other boards, etc.

This makes MicroPython app running in QEMU be pingable from the host (by
following QEMU networking setup instructions,
https://www.zephyrproject.org/doc/samples/net/qemu_setup.html).

uart_irq_input_hook_set() was renamed to uart_console_in_debug_hook_install()
and accepts different params.