conv_linker_config handling existing output file

`proto` sub command now fails when the output file exists.
This is to avoid accidental overwrite.

To handle the case properly, it support --force and --append.

This is to support amending /{partition}/etc/linker.config.pb in the
build process.

Bug: 244531518
Test: manual testing
Change-Id: I0af8c83015e485f2c7533221cae8caf6143403c8