[U-Boot] Commit at91: Choose environment variables location within make config target

[Remy Bohmer]

Hello Nicholas,

The commit mentioned in the subject cause these error messages here
during make at91sam9261ek_config:

These are my steps:
$ export BUILD_DIR=/tmp/build
$ rm -rf /tmp/build
$ make at91sam9261ek_config
/bin/bash: line 7: /tmp/build/include/config.h: No such file or directory
... with environment variable in SPI DATAFLASH CS0
Configuring for at91sam9261ek board...

Is this what you had in mind? Or is this something that you need to fix...

Kind Regards,

Remy