uClibc: use the Buildroot toolchain options instead of the hidden common options

Since we are some day going to finally rename the badly named common
toolchain options (BR2_USE_WCHAR, BR2_ENABLE_LOCALE, BR2_INET_RPC,
etc.) into something more logical, let's start using the Buildroot
toolchain options in the uClibc code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>