Add base module platform Provides so DNF can auto-discover modular platform
    
    In cases where CentOS 8 chroots are being made from scratch and modular
    content will be used as part of the initial install, DNF fails to resolve
    modular dependencies because it cannot discover the modular platform.
    
    DNF does modular platform discovery in two ways:
    
    * Reading the PLATFORM_ID from os-release(5)
    * Reading the package providing system-release for base-module() Provides
    
    The former method works on an existing CentOS 8 environment, but when
    trying to produce a brand new one without an existing reference or
    manual setting of the value, DNF relies on the latter method.
    
    This declaration did not exist, which caused tools like appliance-creator,
    livecd-creator, kiwi, and others to fail to be able to resolve modular dependencies,
    essentially blocking the usage of modules with these tools.
    
    In addition, trivially fix the PLATFORM_ID definition in os-release(5)
    to be set using %base_release_version like the base-module() Provides does.
    
    Reference: https://bugs.centos.org/view.php?id=16877#c35920
    
    Signed-off-by: Neal Gompa <ngompa@centosproject.org>