Apply suggestions from code review

mgbdis.py

@@ -14,7 +14,6 @@
 import os
 import png
 from shutil import copyfile
-from shutil import which
 
 from instruction_set import instructions, cb_instructions, instruction_variants
 
@@ -1171,10 +1170,7 @@ def write_makefile(self):
  else:
  f.write('\trgblink -n game.sym -m game.map -o $@ $<\n')
  f.write('\trgbfix -v -p 255 $@\n\n')
- if which("md5") is not None:
- f.write('\tmd5 $@\n\n')
- else:
- f.write('\tmd5sum $@\n\n')
+ f.write('\t@if which md5sum &>/dev/null; then md5sum $@; else md5 $@; fi\n\n')
 
  f.write('clean:\n')
  f.write('\trm -f game.o game.{} game.sym game.map\n'.format(rom_extension))