devtool: add: support adding a native variant

devtool: add: support adding a native variant

Sometimes you need to build a variant of a recipe for the build
host as well as for the target (i.e. BBCLASSEXTEND = "native"); add a
 --also-native command line option to "recipetool create" that enables
this and plumb it through from an identical option for "devtool add".

(We could conceivably do the same for nativesdk, but I felt it might be
confusing within the context of the extensible SDK, where nativesdk
isn't really relevant to the user.)

(From OE-Core rev: f3bea83db173cce921a9a30f04e88b7e3ed98854)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>