env: Avoid using a leftover text-environment file

If include/generated/environment.h exists (perhaps leftover from a build
of another board) it is used, even if the board currently being built does
not have a text environment.

This causes a build error. Fix it by emptying the file if it should not be
there.

Fixes: https://source.denx.de/u-boot/u-boot/-/issues/9


Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Sean Anderson <seanga2@gmail.com>
Tested-by: Sean Anderson <seanga2@gmail.com>