ROM-Code passes boot source information in r4. First stage does likewise
when invoking second stage. When net booting second stage, r4 has no
definite value. Fix this and pass the original boot source along.

This gives us a valid $bootsource value in net booted barebox, which is
important, so the same environment is loaded.

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