Fix serial driver ， for #243

for @grissiom commented pull request：
“This merge breaks every uart driver in all BSPs. @bright-pan , 
is there any way to get this done without breaking existing code?”

i fix it , using RT_USING_SERIAL_NEW, it is not breaks other BSPs, 
the other BSPs is can use old serial framework.

you can delete old framework easily, After all BSPs has ported.

the patch is works fine.