The dependencies for generating the environment do not work properly:

- If files are removed from the defaultenv, a subsequent make will not
  update the default environment.
- If CONFIG_DEFAULT_ENVIRONMENT_PATH changes, the default environment
  also will not be regenerated.

This patch fixes this by introducing a cmd_env which has the content of
$(ENV_FILES) in the command so that the if_changed mechanism recognizes
a change when $(ENV_FILE) changes. This also results in a nice "  ENV "
string in the build process.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>