malloc: Enable SYS_MALLOC_RUNTIME_INIT by default in SPL

On boards with size restrictions, 1-2k can be a significant fraction of the
binary size. Add a new SPL version of SYS_MALLOC_RUNTIME_INIT. As this
trades text size for BSS size, enable it by default only for boards with at
least 16k of BSS.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>