Add THREAD_FLAGS_EXIT_ON_IRQ for preempted threads

Adds a THREAD_FLAGS_EXIT_ON_IRQ flag to the saved state of a thread
to make it possible to check that no other thread may resume if there's
a thread preempted by an IRQ.
Fix warning in sta_helloworld

* Fixes a couple of printf warnings visible on higher log levels.
* Cleans up core/arch/arm32/sta/sub.mk
* Adds a compiler.h to provide the __unused macro to avoid warnings
  about unused parameters.
GitHub usage documentation

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Removed unused KTA related defines

- Moved IO macro from kta_mem.h to io.h.
- Removed unused TEE_MEM_xyz defines.
- Removed code that was used for TA's in TEE RAM (impossible to reach
  that code).

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Removed file core/include/kernel/kta_mem.h

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Adding GlobalPlatform documentation

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>