Do PD communications in Setup mode

A good first step towards the upcoming 1.1 release, this commit runs the
Power Delivery threads in Setup mode.  This required a slight change to
the shell to make it non-blocking, as otherwise the PD threads would
never get to run.

There's still a lot to do!  The shell and the PD threads fight over
control of the LED in Setup mode.  There's no way to make the PD threads
re-negotiate the required power.  There's also no way to turn the output
on or off from the shell.  None of these changes should be too major,
but together they'll be pretty cool when they're all done.

src/main.c

@@ -61,11 +61,37 @@ static const I2CConfig i2c2config = {
     0
 };
 
+/*
+ * Start the USB Power Delivery threads
+ */
+static void start_pd(void)
+{
+    /* Start I2C2 to make communication with the PHY possible */
+    i2cStart(&I2CD2, &i2c2config);
+
+    /* Initialize the FUSB302B */
+    fusb_setup();
+
+    /* Create the policy engine thread. */
+    pdb_pe_run();
+
+    /* Create the protocol layer threads. */
+    pdb_prlrx_run();
+    pdb_prltx_run();
+    pdb_hardrst_run();
+
+    /* Create the INT_N thread. */
+    pdb_int_n_run();
+}
+
 /*
  * Enter setup mode
  */
 static void setup(void)
 {
+    /* Start the USB Power Delivery threads */
+    start_pd();
+
     /* Indicate that we're in setup mode */
     chEvtSignal(pdb_led_thread, PDB_EVT_LED_CONFIG);
 
@@ -90,22 +116,8 @@ static void setup(void)
  */
 static void sink(void)
 {
-    /* Start I2C2 to make communication with the PHY possible */
-    i2cStart(&I2CD2, &i2c2config);
-
-    /* Initialize the FUSB302B */
-    fusb_setup();
-
-    /* Create the policy engine thread. */
-    pdb_pe_run();
-
-    /* Create the protocol layer threads. */
-    pdb_prlrx_run();
-    pdb_prltx_run();
-    pdb_hardrst_run();
-
-    /* Create the INT_N thread. */
-    pdb_int_n_run();
+    /* Start the USB Power Delivery threads */
+    start_pd();
 
     /* Wait, letting all the other threads do their work. */
     while (true) {

src/shell.c

@@ -393,8 +393,10 @@ bool shellGetLine(BaseSequentialStream *chp, char *line, unsigned size)
         char c;
 
         /* Read a character */
-        if (chSequentialStreamRead(chp, (uint8_t *)&c, 1) == 0)
-            return true;
+        /* The cast to BaseAsynchronousChannel * is safe because we know that
+         * chp is always really of that type. */
+        while (chnReadTimeout((BaseAsynchronousChannel *) chp, (uint8_t *)&c, 1, TIME_IMMEDIATE) == 0)
+            chThdSleepMilliseconds(2);
         /* Abort if ^D is received */
         if (c == '\x04') {
             chprintf(chp, "^D");