Update instructions, fix diff settings and add asm-differ

...to simplify setup a bit

tools/diff_settings.py

@@ -3,6 +3,6 @@
 def apply(config, args):
     config['arch'] = 'aarch64'
     config['baseimg'] = 'data/main.elf'
-    config['myimg'] = 'build/uking.elf'
+    config['myimg'] = 'build/uking'
     config['source_directories'] = ['src']
     config['objdump_executable'] = 'aarch64-linux-gnu-objdump'