config-linux: Transition from Markdown to AsciiDoc

Dropping the "Platform-specific configuration" header so we can down
to:

  6. Container Configuration
    6.7. Linux-specific Container Configuration
      6.7.5. Control groups
        6.7.5.1. Device whitelist

without exceeding AsciiDoc's four-level section limit [1].

[1]: http://asciidoc.org/userguide.html#_sections

Signed-off-by: W. Trevor King <[email protected]>

Author: wking