env: only build env_embedded and envcrc when needed

The env code is protected by the ENV_IS_EMBEDDED define, so attempting to
compile the code when this isn't defined is pointless.  Now that the env
headers have unified around CONFIG_ENV_IS_EMBEDDED, convert the build
system to only build the env objects when this is enabled.  And now that
the env code is conditionally compiled, we can drop the source code checks.

For people who want to extract the environment manually, add a new option
CONFIG_BUILD_ENVCRC that only enables the envcrc utility.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>