[CORE] Only disable runtime module loading/unloading when CONFIG_MODULES=n

Currently, if we disable runtime module loading/unloading (i.e.
CONFIG_MODULES=n) then vmm_modules.c is not compiled at all. This
means no modules will be initialized at boot-time even if the modules
are meant to be built-in. Also, Xvisor ARM crashes because it tries to
call RTC APIs without initializing RTC module.

The linux-compatible meaning of CONFIG_MODULES=n is to only disable
runtime module loading/unloading but built-in modules are still allowed.

This patch selectively compiles out lot of code when CONFIG_MODULES=n
instead of not compiling vmm_modules.c. This saves us roughly 40-50KB
on Xvisor ARM when CONFIG_MODULES=n and also allows built-in modules
when CONFIG_MODULES=n.

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