Add dracut args for /usr to bootloader (#787893)

When /usr is on a separate device like LVM or RAID dracut needs
to know about this so add the needed rd.* parameters to the cmdline.
This also works when upgrading.

pyanaconda/bootloader.py

@@ -900,6 +900,11 @@ def set_boot_args(self, *args, **kwargs):
 
         dracut_devices.extend(self.storage.fsset.swapDevices)
 
+        # Does /usr have its own device? If so, we need to tell dracut
+        usr_device = self.storage.mountpoints.get("/usr")
+        if usr_device:
+            dracut_devices.extend([usr_device])
+
         done = []
         # When we see a device whose setup string starts with a key in this
         # dict we pop that pair from the dict. When we're done looking at