Update documentation/build_system.md

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Align compilation flag optee_os/optee_test(teetest)

Add plat-stm: use -mfloat-abi=soft
to fix VFP register register arguments error during testsuite linking.
It defines GCC to generate output containing library calls for floating-point operations.
optee_test(teetest) is statically linked with new client/linux driver so far.

Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)
Signed-off-by: Cedric Chaumont <cedric.chaumont@st.com>
Clean syscall handling

* Implements a svc handler suitable to supply as a handler for
  thread_svc_handler.
* Removes hardcoded call to tee_svc_sycall in
  thread_svc_handler.
* Removes duplicated code for unwinding of stack after
  tee_svc_enter_user_mode() replacing it with a single
  tee_svc_unwind_enter_user_mode()

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (QEMU virt and FVP)
Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)
Travis: use default arm-linux-gnueabihf- compiler

The download of the Linaro compiler as well as the installation of
its dependencies (including some 32-bit libraries) is replaced by
the installation of the default gcc-arm-linux-gnueabihf package.

This speed up the build time by about a minute.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>