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>

The BootROM passes us information about the boot medium in r4 and we
already use that in first stage and pass it along to second stage
PBL already. To make use of it, we need to pass it to barebox proper, do
this by writing it in the last 4 bytes of the SRAM. As second stage
always run in DRAM, this is safe to do.

We could also write to SRAM directly from first stage, but at91bootstrap
passes info in r4 as well for the sama5d3 boards, so we do it likewise
to maintain compatibility.

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

SAMA5 BootROM passes information about the boot source in the r4
register. Add functions to parse these. To make use of this, entry point
must back up the r4 register, because otherwise it's clobbered by local
variable use.

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