Add simple cooperative threading scheme to allow parallel hw init.

Enable system for running hardware initialization in parallel.
The yield() call can now round-robin between "threads".
Rework ata controller init to use a thread per controller.
Make sure internal drives are registered in a defined order.
Run keyboard initialization in a thread.
Rework usb init to use a thread per controller.
diff --git a/src/boot.c b/src/boot.c
index 668ddb0..d80eae4 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -115,6 +115,23 @@
         return;
 
     struct ipl_entry_s *ie = &IPL.bcv[IPL.bcvcount++];
+    if (CONFIG_THREADS) {
+        // Add to bcv list with assured drive order.
+        struct ipl_entry_s *end = ie;
+        for (;;) {
+            struct ipl_entry_s *prev = ie - 1;
+            if (prev < IPL.bcv || prev->type != BCV_TYPE_INTERNAL)
+                break;
+            struct drive_s *prevdrive = (void*)prev->vector;
+            if (prevdrive->type < drive_g->type
+                || (prevdrive->type == drive_g->type
+                    && prevdrive->cntl_id < drive_g->cntl_id))
+                break;
+            ie--;
+        }
+        if (ie != end)
+            memmove(ie+1, ie, (void*)end-(void*)ie);
+    }
     ie->type = BCV_TYPE_INTERNAL;
     ie->vector = (u32)drive_g;
     ie->description = "";
@@ -297,6 +314,8 @@
     if (! CONFIG_BOOT)
         return;
 
+    // XXX - show available drives?
+
     // Allow user to modify BCV/IPL order.
     interactive_bootmenu();