Remove k command and use dnx instead

Author: ChengTian

build.sh

@@ -31,7 +31,7 @@ if ! type dnvm > /dev/null 2>&1; then
     source packages/KoreBuild/build/dnvm.sh
 fi
 
-if ! type k > /dev/null 2>&1; then
+if ! type dnx > /dev/null 2>&1; then
     dnvm upgrade
 fi