snippets: add serial-console

Add a new snippet to enable serial console. Even though most Zephyr
boards enable this by default, the same is not true for many real
boards, where enabling UART can increase power consumption. Having
this snippet may become handy on such scenarios. We could also
consider cleaning up upstream boards by just enabling this snippet
by default.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>