Add GIT version information (commid ID) to untagged U-Boot versions

As done in the linux kernel, the U-Boot version (U_BOOT_VERSION)
of all unreleased (untagged) U-Boot images will be automatically
extended upon compiletime with a part of the GIT commit ID and
possibly with "dirty" if uncommited changes are detected.

Here an example for the resulting version:
"U-Boot 1.1.4-g3457ac18-dirty"

The version is now maintained in the toplevel Makefile and the
version headers are autogenerated.

Patch by Stefan Roese, 9 Feb 2006