in port/espressif/src/boot_heap_port.c [12:20]
void boot_port_heap_init( void )
{
#ifdef MCUBOOT_USE_MBED_TLS
mbedtls_memory_buffer_alloc_init(crypto_heap, sizeof(crypto_heap));
#else
/* Tinycrypt does not require a heap.
* However, if MCUBoot loader is used, this needs implementation. */
#endif
}