meson: uart: get rid of kernel thread for chars rx

Signed-off-by: Carlo Caione <carlo@caione.org>
meson: uart: clean up

Signed-off-by: Carlo Caione <carlo@caione.org>
meson: uart: rewritten meson_uart_start_tx() and small fixes

Signed-off-by: Carlo Caione <carlo@caione.org>