Use ENABLE_SMP_SUPPORT define

[axel-h]

Follow-up from #95 (comment)

Since the kernel's one private config header are public now (see https://github.com/seL4/seL4/blob/master/libsel4/include/sel4/config.h#L96), the define ENABLE_SMP_SUPPORT can be used in the ELF-Loder code also instead of checking CONFIG_MAX_NUM_NODES > 1. So we could replace this everywhere to align the code.