Serial tweaks

Process incoming data first. Reset link on invalid command.

Signed-off-by: Paul Brook <paul@nowt.org>
Fix reset glitch

Signed-off-by: Paul Brook <paul@nowt.org>
Disable unused peripherals

Signed-off-by: Paul Brook <paul@nowt.org>
Sleep mode

Signed-off-by: Paul Brook <paul@nowt.org>
Deeper sleep when off

Signed-off-by: Paul Brook <paul@nowt.org>