only generate irq headers on demand

as header file generation is not directly controlled by make (which, by
the way, makes the generatedheaders target phony), the script has to
take care of not needlessly generating files itself lest to have make
rebuild everything everytime

scripts/irq2nvic_h

@@ -125,6 +125,10 @@ def makeparentdir(filename):
         # where is my 'mkdir -p'?
         pass
 
+def needs_update(infiles, outfiles):
+    timestamp = lambda filename: os.stat(filename).st_mtime
+    return any(not os.path.exists(o) for o in outfiles) or max(map(timestamp, infiles)) > min(map(timestamp, outfiles))
+
 def main():
     infile = sys.argv[1]
     if not infile.startswith('./include/libopencm3/') or not infile.endswith('/irq.yaml'):
@@ -132,6 +136,9 @@ def main():
     nvic_h = infile.replace('irq.yaml', 'nvic.h')
     vector_nvic_c = infile.replace('./include/libopencm3/', './lib/').replace('irq.yaml', 'vector_nvic.c')
 
+    if not needs_update([__file__, infile], [nvic_h, vector_nvic_c]):
+        sys.exit(0)
+
     makeparentdir(nvic_h)
     makeparentdir(vector_nvic_c)