sw: add printf

We'll use this to print debugging information out the serial port.

Signed-off-by: Sean Cross <sean@xobs.io>

sw/src/usb-cdc.c

@@ -1,4 +1,6 @@
+#include <usb.h>
 #include <usb-cdc.h>
+#include <fomu/csr.h>
 
 static int connected = 0;
 
@@ -10,4 +12,13 @@ int cdc_connected(void)
 void cdc_set_connected(int is_connected)
 {
     connected = is_connected;
+}
+
+void _putchar(char character)
+{
+    // Wait for buffer to be empty
+    while (usb_ep_2_in_respond_read() == EPF_ACK)
+        ;
+    usb_ep_2_in_ibuf_head_write(character);
+    usb_ep_2_in_respond_write(EPF_ACK);
 }