introduce a USE_EXTERNAL_LIBC symbol which gets selected by EXTERNAL_TOOLCHAIN || NATIVE_TOOLCHAIN Currently we always assume uClibc if an external toolchain is used, this breaks for non-uClibc toolchains or even vanilla uClibc ones since they do not share the external librpc semantics as OpenWrt. Solve the problem by defining an abstract "EXTERNAL_LIBC" which packages might or might not depend on.

SVN-Revision: 29689