Version 25.2.3 (and higher) of Android SDK Tools package contains new tool - sdkmanager - which simplifies this task of installing build-tools from the command line.
It is located in android_sdk/tools/bin folder.
Usage (from documentation):
List installed and available packages:
sdkmanager --list [options] \
      [--channel=channel_id] // Channels: 0 (stable), 1 (beta), 2 (dev), or 3 (canary)
Use the channel option to include a package from a channel up to and including channel_id. For example, specify the canary channel to list packages from all channels.
Install packages:
sdkmanager packages [options]
The packages argument is an SDK-style path, wrapped in quotes (for
example, "build-tools;25.0.0" or "platforms;android-25"). You can
pass multiple package paths, separated with a space, but they must
each be wrapped in their own set of quotes.
Example usage (on my Mac):
alex@mbpro:~/sdk/tools/bin$ ls ../../build-tools/  
25.0.0/   
alex@mbpro:~/sdk/tools/bin$ ./sdkmanager "build-tools;25.0.2"  
done   
alex@mbpro:~/sdk/tools/bin$ ls ../../build-tools/  
25.0.0/ 25.0.2/
You can also specify various options, for example to force all connections to use HTTP (--no_https), or in order to use proxy server (--proxy_host=address and --proxy_port=port).
To check the available options, use the --help flag. On my machine (Mac), the output is as following:
alex@mbpro:~/sdk/tools/bin$ ./sdkmanager --help
Usage: 
  sdkmanager [--uninstall] [<common args>] \
    [--package_file <package-file>] [<packages>...]
  sdkmanager --update [<common args>]
  sdkmanager --list [<common args>]
In its first form, installs, or uninstalls, or updates packages.
    <package> is a sdk-style path (e.g. "build-tools;23.0.0" or 
             "platforms;android-23").
    <package-file> is a text file where each line is a sdk-style path
                   of a package to install or uninstall.
    Multiple --package_file arguments may be specified in combination
     with explicit paths.
In its second form (with --update), currently installed packages are
    updated to the latest version.
In its third form, all installed and available packages are printed out.
Common Arguments:
    --sdk_root=<sdkRootPath>: Use the specified SDK root instead of the SDK containing this tool
    --channel=<channelId>: Include packages in channels up to <channelId>.
                           Common channels are:
                           0 (Stable), 1 (Beta), 2 (Dev), and 3 (Canary).
    --include_obsolete: With --list, show obsolete packages in the
                        package listing. With --update, update obsolete
                        packages as well as non-obsolete.
    --no_https: Force all connections to use http rather than https.
    --proxy=<http | socks>: Connect via a proxy of the given type.
    --proxy_host=<IP or DNS address>: IP or DNS address of the proxy to use.
    --proxy_port=<port #>: Proxy port to connect to.
* If the env var REPO_OS_OVERRIDE is set to "windows",
  "macosx", or "linux", packages will be downloaded for that OS.