kbuild: deb-pkg: hide KDEB_SOURCENAME from Makefile

scripts/Makefile.package does not need to know the value of
KDEB_SOURCENAME because the source name can be taken from
debian/changelog by using dpkg-parsechangelog.

Move the default of KDEB_SOURCENAME (i.e. linux-upstream) to
scripts/package/mkdebian.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>