Makefile.inc1: Set DISTDIR in stagekernel target

The distributekernel target expects DESTDIR and DISTDIR to be set. The
stagekernel target invokes make distributekernel, and previously left
DISTDIR unset, resulting in a path with a "//" component. Instead, set
DISTDIR to . to make the way we're (ab)using the distributekernel target
more explicit.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48288