libc_mem_support - Heap Area cannot be changed at startup

The Heap area is defined through linker file. When using SDRAMs other than Hilscher defaults (larger than 8MB) at least the upper area needs to be changeable.

Proposal: Add parameters to libc_mem_init() that define the heap and let the user call it before entering rcX kernel. Once initialized the further requests inside rcX startup should be ignored.