busybox: alternatives: accomodate CONFIG_BUSYBOX_CUSTOM=y

When busybox customisation is enabled, we should depend on config
symbols CONFIG_BUSYBOX_CONFIG_xxx to form alternatives specs

Reported-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>