common, ubi: use positive return values for ubi check

The ubi check command is expected to not fail and just check whether
a volume exist or not. Currently, when a volume does not exist, the
command fails which leads to an error:
"exit not allowed from main input shell."

Use 1 to indicate that a volume does not exist. This allows to use
ubi check in an if statement, e.g.
if ubi check rootfs; then; echo "exists"; else; echo "not there"; fi