env: delete selected vars not present in imported env

When variables explicitly specified on the command line are not present
in the imported env, delete them from the running env.
If the variable is also missing from the running env, issue a warning.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Reviewed-by: Marek Vasut <marex@denx.de>