classes: Add recipe class to overrides

classes: Add recipe class to overrides

We have currently no override to detect a recipe being build cross, crosssdk
or for target at times we can use virtclass-native and virtclass-nativesdk to
override stuff in recipes but we dont have way to modify a variables
based on recipe type always.

This patch adds in such an override and in particular makes a target override
class available.

With this change now we can say:

EXTRA_OECONF_class-target = "...."
EXTRA_OECONF_class-native = "..."
EXTRA_OECONF_class-nativesdk = "..."
EXTRA_OECONF_class-crosssdk= "..."

Based of an original patch by Khem Raj

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>