kbuild: tar-pkg with out-out-tree building

Fix out-of-tree builds for the tar-pkg targets

When I wrote the buildtar script, I didn't even think about
out-of-tree builds because I didn't use these back then. This patch
throughoutly uses ${objtree} instead of `pwd`.

Also, the kernel version is no longer manually built. Instead, it will
properly use $KERNELRELEASE .  Installing modules is only done if
CONFIG_MODULES is set.

Signed-off-by: Jan-Benedict Glaw <jbglaw@lug-owl.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>