unify {CONFIG_,}ENV_IS_EMBEDDED

Some boards have fallen out of sync by defining CONFIG_ENV_IS_EMBEDDED
manually.  While it is useful to have this available to the build system,
let's do it automatically rather than forcing people to opt into it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>