Specify len() of object by converting to list

Helps resolve the following error in python3:
TypeError: object of type 'filter' has no len()

Change-Id: Id607186baeacc3802f651d5cb72af82d0c10115b
Signed-off-by: Saqib Khan <khansa@us.ibm.com>
diff --git a/classes/obmc-phosphor-utils.bbclass b/classes/obmc-phosphor-utils.bbclass
index baaa19d..8ac8c7d 100644
--- a/classes/obmc-phosphor-utils.bbclass
+++ b/classes/obmc-phosphor-utils.bbclass
@@ -26,7 +26,7 @@
 
 
 def listvar_to_list(d, list_var, sep=' '):
-    return filter(bool, (d.getVar(list_var, True) or '').split(sep))
+    return list(filter(bool, (d.getVar(list_var, True) or '').split(sep)))
 
 
 def compose_list(d, fmtvar, *listvars, **kw):
@@ -49,4 +49,4 @@
     newval = []
     for w in words:
         newval.append(w + suffix)
-    return ' '.join(newval)
\ No newline at end of file
+    return ' '.join(newval)