Add 'merge' command to conv_linker_config

'merge' command can merge multiple configurations.
It just delegates protobuf's msg.MergeFrom(other_msg).

  usage: conv_linker_config merge -o out.pb -i a.pb -i b.pb ...

out.pb will have all keys from multiple inputs.

Bug: 181093750
Test: m
Test: MicrodroidTestCase
Change-Id: Ibf715aa0bcc1e5c82c85a9af8fe7dca8d6ab68ad