[BUG] include/generated/environment.h included incorrectly

Hi Simon,

include/generated/environment.h is still included if it exists, even if

CONFIG_USE_DEFAULT_ENV_FILE is not set. To reproduce this, build a board

which uses this config (such as sandbox), and then build a board which

does not. You should see the following error:

In file included from env/common.c:32:

include/env_default.h:113:3: error: #error "Your board uses a text-file environment, so must not define CONFIG_EXTRA_ENV_SETTINGS"

113 | # error "Your board uses a text-file environment, so must not define CONFIG_EXTRA_ENV_SETTINGS"

| ^~~~~

--Sean