serial gadget: enable/disable on request ...

- add a usbserial command to enable/disable the serial gadget
- allow dfu and usbserial to cohexist in the same barebox
- add a timeout in u_serial so that we don't get locked if the user
enable usbserial from a UART console but doesn't consume the data
on the usbserial port created on the PC
- remove debug or verbose printf
- tested on i.MX25 & i.MX35 & usb-a926x

Signed-off-by: Eric Bénard <eric@eukrea.com>
Tested-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>