Kconfig: Move SERIAL, USBSERIAL, and CANSERIAL definitions to src/Kconfig

There is no need to define these options in every board Kconfig file.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/Kconfig

@@ -42,6 +42,8 @@ source "src/linux/Kconfig"
 source "src/simulator/Kconfig"
 
 # Generic configuration options for serial ports
+config SERIAL
+    bool
 config SERIAL_BAUD
     depends on SERIAL
     int "Baud rate for serial port" if LOW_LEVEL_OPTIONS
@@ -51,6 +53,8 @@ config SERIAL_BAUD
         to 250000. Read the FAQ before changing this value.
 
 # Generic configuration options for USB
+config USBSERIAL
+    bool
 config USB_VENDOR_ID
     default 0x1d50
 config USB_DEVICE_ID
@@ -74,6 +78,8 @@ config USB_SERIAL_NUMBER
 endmenu
 
 # Generic configuration options for CANbus
+config CANSERIAL
+    bool
 config CANBUS_FREQUENCY
     int "CAN bus speed" if LOW_LEVEL_OPTIONS && CANSERIAL
     default 500000