Make ESYS heap scheme configurable

[AndreasFuchsTPM]

Since some embedded platforms might not support standard malloc() it would be nice to have an ESYS_MALLOC (and ESYS_REALLOC and ESYS_FREE and ESYS_CALLOC) global variable that can be used by applications to override the allocation scheme.

Question: At compiletime or a runtime ? (Let's look at Mbed-TLS and OpenSSL for an answer; tend towards compiletime)