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

[Reinhard Meyer]

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