toolchain-shar-extract.sh: proper fix for additional env setup scripts

toolchain-shar-extract.sh: proper fix for additional env setup scripts

buildtools-tarball uses a custom env setup script, which isn't named the
same as the default; thus unfortunately OE-Core revision
a36469c97c9cb335de1e95dea5141038f337df95 broke installation of
buildtools-tarball. Revert that and implement a more robust mechanism.

(From OE-Core rev: 00e081b81ba8118959b724269ba9d18d42aba8a4)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>