[oe] USE flags mumbling

[Graeme Gregory]

We already have BBCLASSEXTENDS which modifies ${PN} of a package and
can use overrides to change behaviors of recipes.

Maybe USE flags could be implemented in a similar fashing.

DISTRO_USE = "nossl nox11"

EXTRA_OECONF_append_use-nossl = "--disable-ssl"

${PN} of the recipe becomes XXXX-nossl

Thoughts?

Graeme