/dev/random can block long after boot time. It seems there's a consensus
that /dev/urandom is safe to use except for very early boot, which isn't
when barebox sandbox is usually run. To make the HWRNG more useful,
always use /dev/urandom.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>