input: add some basic debugging utilities

Add two input subsystem options: one for dumping input events, one for
triggering input reports from the shell.

The two are independent from each other, so dumping can be enabled on
any application that has no shell, but if the shell command is present
logging can be toggled on and off with a shell command.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>