Reentrancy fixes

Before this patch: The normal world was only allowed to enter
secure world with one thread at a time.

After this patch: The normal world may try to enter secure world
with as many threads as it likes, secure world will return busy
when no more threads can be allowed. Secure world still only allows
one active thread at a time, but during RPC another thread may enter
and do some work. This is needed for cancellation to work.

* Adds a mutex that waits in normal world if busy
* Adds a new RPC service to wait in normal world
* Imports bitstring.h from FreeBSD to aid mutex implementation
* Adds a critical section in tee_ta_init_session
* Unmaps TA before RPC exit and maps it again on return to handle
  rescheduling of threads during RPC
* Doesn't clear a1-a3 when returning busy
* Bugfixes vector_std_smc_entry

This patch depends on the "Allow parallel entries to secure world"
patch in optee_linuxdriver.