The wic command can be used externally but for this to work, the wic
environment file needs to be present. Therefore write this out
universally, it runs in parallel with other image construction so
any performance implications are negligible.
(From OE-Core rev: b2576f2eab10e4c5dd86449312b417a269cc578e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>