lib/os/heap: option for optimizing for one heap size on 32-bit systems

The "small" heap is is way sufficient for most 32-bit systems.

Let's provide the option to have only one type of heap allowing for
smaller and faster heap code due to not having a bunch of runtime
conditionals based on the heap size.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>