minor fixes on alu

cpu/alu/alu.sv

@@ -47,6 +47,7 @@ always @(*) begin
         alu_out = tmpOut;
     end
 
+    //TODO: improve this part using increment/decrement logic
     if (alu_out < 0)
         flag = 0;   // negative flag
     else if (alu_out = 0)

cpu/cpu.sv

@@ -0,0 +1 @@
+import alu::*;