SE API: minor changes according to comments

- merge tee_svc_open_basic_channel() and tee_svc_open_logical_channel() to
  tee_svc_open_channel()
- remove #ifdef in utee_syscalls_asm.S and put stub function that returns
  TEE_ERROR_NOT_SUPPORTED when SE API is disabled
- rename protocol.[ch] to iso7816.[ch]
- prefix aid_* with "tee_se_"
- add an option to enable/disable se_api_self_tests
- remove unused comments