toolchain-external: define PREFERRED_PROVIDER for linux-libc-headers, but comment it. Someone actually using this needs to verify
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}gcc-intermediate = "external-toolchain-${TOOLCHAIN_BRAND}"
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}gcc = "external-toolchain-${TOOLCHAIN_BRAND}"
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}g++ = "external-toolchain-${TOOLCHAIN_BRAND}"
+#PREFERRED_PROVIDER_linux-libc-headers = "external-toolchain-${TOOLCHAIN_BRAND}"
+
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}gcc-intermediate = "gcc-cross-intermediate"
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}gcc = "gcc-cross"
PREFERRED_PROVIDER_virtual/${TARGET_PREFIX}g++ = "gcc-cross"
+PREFERRED_PROVIDER_linux-libc-headers = "linux-libc-headers"