Refactor mender-convert to use "make install" to install mender

Acceptance criteria:

• Mender-convert must use

    make prefix=DIR install
    

  to install mender and its associated files

• If it's undesirable to call the above because it will compile the client, it is also an option to call single makefile targets, such as

    make prefix=DIR install-identity-scripts install-inventory-scripts install-systemd
    

  Consult the Makefile for the exact targets that should be included