common: fix detection of SYS_MALLOC_F_LEN=0x0

CONFIG_$(SPL_TPL_)SYS_MALLOC_F_LEN is defined as hex. If set to zero
manually, .config contains '0x0' and not '0' as value.

The default value for CONFIG_SPL_SYS_MALLOC_F_LEN should not be set to 0
but to 0x0 if CONFIG_SPL_FRAMEWORK=n to match a manually set value.

Fixes: c0126bd8 ("spl: Support bootstage, log, hash and early malloc in TPL")
Fixes: b6169470 ("SPL: Do not enable SPL_SYS_MALLOC_SIMPLE without SPL_FRAMEWORK by default")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>