Set version to 2.28, not a relase.
    
    bfd/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* version.m4: Bump version to 2.28.0
    	* configure: Regenerate.
    
    binutils/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* configure: Regenerate.
    
    gas/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* configure: Regenerate.
    
    gprof/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* configure: Regenerate.
    
    ld/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* configure: Regenerate.
    
    opcodes/
    2017-03-02  Tristan Gingold  <gingold@adacore.com>
    
    	* configure: Regenerate.