buildman: Use out-env for environment output

buildman: Use out-env for environment output

At present the environment used by U-Boot is written to the 'env'
directory. This is fine when the output directory is not the same as the
source directory, but when it is (as with -w) it conflicts with the source
directory of the same name.

Rename 'env' to 'out-env' to fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>