WITH_META_MODE: Allow buildkernel to create .meta files with curdir==objdir.

WITH_META_MODE: Allow buildkernel to create .meta files with curdir==objdir.

Without this the incremental build was broken since .depend.* are not
generated with .MAKE.MODE=meta and .meta files were not created to
track dependencies.  Typically meta mode does not create .meta files
when building with curdir==objdir but the kernel build is special.

Reported by:	Nikolai Lifanov <lifanov@mail.lifanov.com>
Sponsored by:	EMC / Isilon Storage Division