[CORE] Don't compile vmm_modules if CONFIG_MODULES is not set

Also changed MODULES to CONFIG_MODULES in the menuconfig.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[TOOLS] openconf: Depend on CONFIG_MODULES symbol instead of MODULES

We should be using CONFIG_MODULES symbol for runtime loadable module
for consistency accross Xvisor code hence we update openconf to look
for CONFIG_MODULES symbol instead of MODULES symbol for tristate
options.

Signed-off-by: Anup Patel <anup@brainfault.org>
[ARCH] Rename MODULES=y to CONFIG_MODULES=y in all defconfigs

Signed-off-by: Anup Patel <anup@brainfault.org>