[toolchain] introduce a USE_EXTERNAL_LIBC symbol which gets selected by EXTERNAL_TOOL...

[toolchain] 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.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29689 3c298f89-4303-0410-b956-a3cf2f4a3e73