Makefile.inc1: Forward on METALOG and DISTBASE for kernel targets

Currently IMAKE_INSTALL, which includes -M METALOG, is enough for the
sub-makes to work, but using kldxref for -DNO_ROOT builds will require
manually adding linker.hints to the METALOG, and thus both METALOG
itself and DISTBASE need to be exposed directly to the sub-makes, so do
so.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D43050