Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.
The code was dereferencing 0x800 and loading a value from there, trying to
use a literal value (not address) defined in the linker script
(_ram_fs_cache_block_size) which was 0x800.