toolchain/uclibc: Fix unmanned configuration with "V=s"

Currently on attempt to configure uClibc with "V=s" user is prompted
to answer on configuration questions. Major inconvenience here is
build procedure get paused until user answers all questions.
That happens because not all symbols are defined in our .config.

Solution is as simple as usage of "olddefconfig" target instead of
simple "oldconfig". In that case default values get silently set
for symbols not defined by our .config explicitly.

We haven't noticed that subtle issue before because without "V=s"
what we do is "make oldconfig < /dev/null" which automatically
answers all questions :)

Signed-off-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>