'RELENG_6_0_0_RELEASE'.

This commit was manufactured to restore the state of the 6.0-RELEASE image.

(modload and friends are sufficiently small to consider their inclusion
in PicoBSD images).

in crunch.conf -- the previous choice, CWD, was too subject to false
matches (this string will be replaced by the absolute pathname
during the build process).

tree. Unfortunately the latter cannot be completely readonly, because
"config" still depends on the kernel config file being in sys/${ARCH}/conf
(it seems to derive other pathnames from that one).

so as to make the "picobsd" script less version-specific.

Improve handling of cross-builds (which requires creation of
includes and libraries for the new source tree).

The "picobsd" script will not probably work on -current because it still
uses 'vn' instead of 'md', but i am commiting it anyways to keep it
in sync with the version in -stable.

This does not apply to RELENG_4 for which the compile directory is
../compile/<filename>

There might be some other bug related to OBJDIR handling, once
fixed these changes can be MFC immediately.

better place to handle dependencies.

Make another step at helping cross-compiling: when the user specifies
an alternate source tree, the script takes care of creating include
files and libraries for the new tree.
Furthermore, build and use a version of the "config" program which
matches the new sources.

It takes a long time to create libraries, and it might even not do
the right thing at once, there might be some dependencies that i
have forgotten. At any rate, with this code i have been able to
build a working picobsd image using -CURRENT sources on -STABLE

MFC after: 3 days