[U-Boot] A small hickup when switching over to out-of-tree building

Hello,

when switching from in-tree building to out-of-tree building (BUILD_DIR=<path>) the files *_autogenerated.h left in the in-tree include directory will be included instead of the files in <path>/include. That will cause a newly built u-boot to display some old date and version!

One has to remove them manually or do a "make clean" without BUILD_DIR set to fix the issue.

Best Regards, Reinhard