samples: console: echo: Test output-only mode too

We have cases with UART drivers where TX interrupt handling is not
bootstrapped properly on enabling them. To make such cases more
obvious, start this sample with console_write(), the output of which
wouldn't appear if that's the case. (Of course, the sample starts
with printk() exlaining what may happpen.)

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>