Setting of a variable PREFERRED_PROVIDER_virtual/libc only if it doesn't have a value
(From OE-Core rev: d0e9f74e3f1322b58b78a9bc82f299d6b9da036f)
Signed-off-by: Andrey Belous <abelous@broadcom.com>
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>