fix jffs2reset on ubifs overlay

change find_mount_point parameter to match either all or only
jffs2 and ubifs, which are typically used on embedded mtd devices.
For now, find_mount_point was used without the fstype parameter
except for a single occurance in jffs2reset. There it is desireable
to apply the same logic as previously hard-coded to jffs2 also in the
case of a ubifs overlay, so firstboot aka. factory-reset works as
expected.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>