cmd: nvedit: check for ENV_SUPPORT

Avoid error messages when SPL,TPL,VPL build don't
have the environment options of the main build.
This is needed when defined(CONFIG_ENV_IS_IN_xxx) is changed
to CONFIG_IS_ENABLED(ENV_IS_IN_xxx).

Signed-off-by: Troy Kisky <troykiskyboundary@gmail.com>