console_get_by_name iterates through the console_list using strcmp to mach
a given console by name. console_list may contain entries with devname = NULL,
inserted there e.g. by KEYBOARD_GPIO. In worst case loady -t usbserial
will crash barebox when strcmp hits 0 NULL devname:
> unable to handle NULL pointer dereference at address 0x00000000

Let console_get_by_name just ignore the anonymous consoles.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>