Append the shortened git revision to the u-boot version

This add the first 7 characters of the git sha-1 to the compiled in
u-boot version.

With  this change the serial console displays the following version information:

U-Boot 2014.04-gee425f9 (Oct 01 2014 - 22:20:35) Allwinner Technology

Without the version looks like this:

U-Boot 2014.04 (Oct 01 2014 - 21:55:57) Allwinner Technology


Signed-off-by: Christian Ege <k4230r6@gmail.com>