malloc: Don't statically initialize av_ if using malloc_init

When we enable malloc_init, there is no need to statically initialize
av_, since we are going to do it manually. This lets us move av_ to
.bss, saving around 1-2k of data (depending on the pointer size).

cALLOc must be adjusted to not access top before malloc_init.

While we're at it, rename/reword the Kconfig to better describe what
this option does.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>