gentpl.py: Ensure output is deterministic

The output of the SOURCES lines in grub-core/Makefile.core.am, generated
from grub-core/Makefile.core.def with gentpl.py is not deterministic due to
missing sorting of the list used to generate it. Add such a sort.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

gentpl.py

@@ -569,6 +569,7 @@ def foreach_platform_value(defn, platform, suffix, closure):
     for group in RMAP[platform]:
         for value in defn.find_all(group + suffix):
             r.append(closure(value))
+    r.sort()
     return ''.join(r)
 
 def platform_conditional(platform, closure):