bootstd: rauc: Only scan all partitions instead of boot files

bootstd: rauc: Only scan all partitions instead of boot files

Only scan for the existence of all required partitions of a RAUC system,
instead of searching for boot script files in all of them.

Previously, it might have occurred, that a slot did not contain required
files and RAUC already marked the corresponding slot as bad (not
suitable for booting). In that case, scanning for a non-existence boot
script would result in an error (and thus not booting anything), which
was different behavior compared to the legacy RAUC boot.

Signed-off-by: Martin Schwan <m.schwan@phytec.de>