Improve control of debug messages.

Rename BX_INFO() to dprintf() and add a "severity level" parameter.
Add CONFIG_DEBUG_LEVEL compile option to control debug verbosity.
Add more debug info to init steps of post.c.
diff --git a/src/boot.c b/src/boot.c
index 12226d6..336d869 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -96,7 +96,7 @@
     bootdev -= 1;
 
     if (bootdev >= GET_EBDA(ipl.count)) {
-        BX_INFO("Invalid boot device (0x%x)\n", bootdev);
+        dprintf(1, "Invalid boot device (0x%x)\n", bootdev);
         return;
     }
 
@@ -174,7 +174,7 @@
     }
 
     /* Debugging info */
-    BX_INFO("Booting from %x:%x\n", bootseg, bootip);
+    dprintf(1, "Booting from %x:%x\n", bootseg, bootip);
 
     memset(&cr, 0, sizeof(cr));
     cr.ip = bootip;