Merge ~mwhudson/ubiquity:lp-1847898 into ubiquity:master

[Michael Hudson-Doyle]

[Michael Hudson-Doyle (mwhudson)]

I haven't tested this yet.

[Michael Hudson-Doyle (mwhudson)]

I have now tested this and it worked for me.

[Dimitri John Ledkov (xnox)]

review: Approve

diff --git a/ubiquity/misc.py b/ubiquity/misc.py
index c0b0536..ad40a49 100644
--- a/ubiquity/misc.py
+++ b/ubiquity/misc.py
@@ -371,11 +371,23 @@ def grub_default(boot=None):
 
     devices = grub_device_map()
     target = None
-    if devices:
+
+    if boot is not None:
+        for device in devices:
+            try:
+                candidate = os.path.realpath(device.split('\t')[1])
+            except (IndexError, OSError):
+                pass
+            if candidate == boot:
+                target = candidate
+                break
+
+    if target is None and devices:
         try:
             target = os.path.realpath(devices[0].split('\t')[1])
         except (IndexError, OSError):
             pass
+
     # last resort
     if target is None:
         target = '(hd0)'