scripts/package: add KBUILD_PKG_ROOTCMD variable

Let the deb-pkg target acquire (fake) root privileges before
running commands that need them.  Without such privileges,
deb-pkg errors out because chown fails.

The new KBUILD_PKG_ROOTCMD variable, if defined, is used as a
command to run other commands with possibly fake elevated
privileges.  Since this is not needed for the tar-pkg and rpm-pkg
targets, it is only used by deb-pkg.  If it is not defined, the
behavior is as before, and the user will have to rerun make as
root.

In other words, as a shortcut, instead of running 'make oldconfig &&
make && fakeroot -u make deb-pkg', one can use the single command
'make oldconfig deb-pkg KBUILD_PKG_ROOTCMD="fakeroot -u"'.

Suggested-by: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>