kbuild: Do not overwrite makefile as anohter user

Change the conditional of the outputmakefile rule to be evaluated entirely
in make, and add a conditional to not touch the generated makefile when e.g.
running 'make install' as root while the build was done as non-root. Also
adjust the comment describing this, and move the message printing and
redirection to mkmakefile.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>