[OE-core] pkgconfig vs pkgconf

[Mark Hatle]

As of this morning, I'm not getting a warning about a preferred provider being
necessary to differentiate between pkgconfig and pkgconf.

I've set the following to make it "go away".  But I assume we need to set a
default somewhere in the system...

PREFERRED_PROVIDER_pkconfig = "pkgconfig"
PREFERRED_PROVIDER_pkgconfig-native = "pkgconfig-native"

PREFERRED_RPROVIDER_pkgconfig = "pkgconfig"