spl: Create proper symbols for enabling the malloc() pool

For U-Boot proper we have CONFIG_SYS_MALLOC_F which indicates that a
malloc() pool is available before relocation.

For SPL we only have CONFIG_SPL_SYS_MALLOC_F_LEN which indicates the
size of the pool.

In various places we use CONFIG_SPL_SYS_MALLOC_F_LEN == 0 to indicate
that there is no pool.

This differing approach is confusing. Add a new CONFIG_SPL_SYS_MALLOC_F
symbol for SPL (and similarly for TPL and VPL). Tidy up the Kconfig
help for clarity.

For now these symbols are not used. That is cleaned up in the following
patches.

Signed-off-by: Simon Glass <sjg@chromium.org>