busybox: set/unset CONFIG_NOMMU

Set busybox .config CONFIG_NOMMU appropiately and streamline this new
setting with the previous BUSYBOX_DISABLE_MMU_APPLETS in a single
BUSYBOX_SET_MMU definition (since it's not just applets now).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>