fsl-u-boot-localversion: Easy LOCALVERSION handling

fsl-u-boot-localversion: Easy LOCALVERSION handling

This class allow for easy extending of U-Boot recipes to easy the
LOCALVERSION handling, allowing to reuse of code between different
U-Boot recipes

The following options are supported:

 SCMVERSION        Puts the Git hash in U-Boot local version
 LOCALVERSION      Value used in LOCALVERSION (default to '+fslc')

This about avoid code duplication among Layerscape and i.MX.

Change-Id: Icec618092d1f70606051619b6f6118f50e7f1ba5
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>