bootstd: rauc: Change global method to check any partition

bootstd: rauc: Change global method to check any partition

The bootmeth rauc should scan all partitions, in particular whole
devices, and not be a global method. There may exist multiple RAUC
systems on different devices and they should all be detected. This also
fixes a bug, where both a global bootflow and one using an actual,
complete device would be detected at the same time, when scanning for
valid bootflows.

Signed-off-by: Martin Schwan <m.schwan@phytec.de>
Tested-by: Wadim Egorov <w.egorov@phytec.de>