travis: Don't copy files into .bm-work/

At present if TEST_PY_BD is empty the script copies various files into a
directory, to no purpose. This happens because UBOOT_TRAVIS_BUILD_DIR is
set before TEST_PY_BD is tested.

Move the 'if' to fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>