[scripts] add ext-toolchain.sh, a utility for external toolchains

[scripts] add ext-toolchain.sh, a utility for external toolchains
The ext-toolchain.sh utility script implements various external toolchain tasks:
  * Testing of available features, like c++, soft-float or ipv6 support
  * Finding the libc implementation (uclibc or eglibc/glibc)
  * Inferring the GNU target name
  * Finding shared objects for packaging (libc, libpthread, ...)
  * Finding executables for packaging (ldd, gdbserver, ...)
  * Generating wrapper scripts for toolchain commands

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29765 3c298f89-4303-0410-b956-a3cf2f4a3e73