mkimg: bump version to 20151211 after r292082

mkimg has had a number of functional additions after the last time the
version was incremented. Do so now, to r292082's commit date, so that
users can determine what is supported.

Reviewed by: marcel
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6882