mtd: OneNAND: Allow setting of boundary information when built as module

This patch unifies the flex_bdry setting for module vs. built-in
configuration of OneNAND.

Signed-off-by: Amul Kumar Saha <amul.saha@samsung.com>
Signed-off-by: Vishak G <vishak.g@samsung.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>