1. fix unsupported cantilever was not warned. We need to set support
type to stTreeAuto in is_support_necessary() to fully utilize the
power of feature detection.
2. fix tree support not generated on non-first plate.
Change-Id: I5b8aa827c9d62df0119db4e8e9212f33ce75ddaa
(cherry picked from commit 60f21b6fcb45fe56f7fc867a7fcad433282bc055)