test: env: add test for u-boot-initial-env creation

Checks that `make u-boot-initial-env` creates the text file
u-boot-initial-env and checks that it at least contains
`board=<something>`.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>