firmware: do not use relocated _boot_status before it is valid

When OpenSBI is started from an address not equal to the link address,
it is first relocated to the link address. Hart 0 performs the
relocation and notifies the other harts of its completion with the
_boot_status variable. It uses the copy of the variable relative to the
link address. This copy contains valid data only after relocation has
finished. The waiting harts will therefore read invalid data until
relocation has finished. This can cause them to continue execution too
early.

Fix this by using the _boot_status variable relative to the load address
while OpenSBI has not finished relocation.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup.patel@wdc.com>