You could try new csp 0.1.2, have fix this bug, thank you!

Add ( source "$BSP_DIR/drivers/Kconfig" ) in the project folder's Kconfig to load driver folder's Kconfig, such as:

I have submit csp for samd21 serials, you could try it now.

You should open the setting, un-click & click the Enable SERCOM3 USART checkbox. Save setting, it will add

#define BSP_USING_UART
#define BSP_USING_SERCOM3_USART

in rtconfig.h

Should compile success now