package/optee-*: packages depend on thread support

package/optee-*: packages depend on thread support

BR2_PACKAGE_OPTEE_CLIENT depends on BR2_TOOLCHAIN_HAS_THREADS.
Library teec uses pthread support to protect clients state management.

This change declares this dependency in package optee-client and
updates dependent packages accordingly: optee-benchmark, optee-client,
optee-examples and optee-test.

Fixes [1] and [2] where config selected an toolchain without thread
support and build failed with trace like:

  CMake Error at /home/buildroot/autobuild/run/instance-1/output/host/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
    Could NOT find Threads (missing: Threads_FOUND)
  Call Stack (most recent call first):
    /home/buildroot/autobuild/run/instance-1/output/host/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
    /home/buildroot/autobuild/run/instance-1/output/host/share/cmake-3.8/Modules/FindThreads.cmake:212 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
    libteec/CMakeLists.txt:8 (find_package)

[1] http://autobuild.buildroot.net/results/ed6ffe2197da4f3a970bd3c5522291236396cc8e
[2] http://autobuild.buildroot.net/results/406f90048db097580b626ef889823132f8676ba1

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[Thomas: fix dependencies of Config.in comment, as noticed by Baruch.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>