kbuild: deb-pkg: use default string when variable is unset or null

${DEBFULLNAME-${user}} falls back to ${user} when DEBFULLNAME is unset.
It is more reasonable to do so when DEBFULLNAME is unset or null.

Otherwise, the command:

  $ DEBFULLNAME= make deb-pkg

will leave the name field blank.

The same applies to KBUILD_BUILD_USER and KBUILD_BUILD_HOST.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>

scripts/package/mkdebian

@@ -125,12 +125,12 @@ gen_source ()
 rm -rf debian
 mkdir debian
 
-user=${KBUILD_BUILD_USER-$(id -nu)}
-name=${DEBFULLNAME-${user}}
+user=${KBUILD_BUILD_USER:-$(id -nu)}
+name=${DEBFULLNAME:-${user}}
 if [ "${DEBEMAIL:+set}" ]; then
 	email=${DEBEMAIL}
 else
-	buildhost=${KBUILD_BUILD_HOST-$(hostname -f 2>/dev/null || hostname)}
+	buildhost=${KBUILD_BUILD_HOST:-$(hostname -f 2>/dev/null || hostname)}
 	email="${user}@${buildhost}"
 fi
 maintainer="${name} <${email}>"