cycle_counters: add id to avr_cycle_counter_t

Add id to avr_cycle_counter_t struct, to be able to differentiate
counters without using string comparison.
tests: update cycle counters test case

Update cycle counters test case to reflect the addition of the id field.
mmcu: add SIMAVR_CMD_USER

Add enum label SIMAVR_CMD_USER to denote start of user-defined commands.