Refactor: Offer single-step library abstraction

[jovanbulck]

SGX_STEP_PLATFORM and SGX_STEP_TIMER_INTERVAL and actual timer configuration AEP code should better be encapsulated in libsgxstep plus a common Makefile.config, such that the actual application AEP callback code can focus exclusively on gathering side-channel information.

[jovanbulck]

single-stepping code is messy now -- applications have to copy the skeleton from app/bench/main.c and directly use libsgxstep's low-level apic and page table manipulations to build the single-stepping primitive each time in the attack application.

Instead, libsgxstep should offer a convenient start/stop_single_step() abstraction that encapsulates all of the low-level details and platform-specific code. At this point we could also take care to enable/disable the APIC timer and pin to a specific CPU core etc.