llext: enable tristate Kconfig options

kconfiglib.py has a hard dependency on CONFIG_MODULES to support 'm'
values for tristate Kconfig options. It's a logical companion for
but can also be used with other configurations.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

subsys/Kconfig

diff --git a/subsys/Kconfig b/subsys/Kconfig
index f0dab7c..2e083fb 100644
--- a/subsys/Kconfig
+++ b/subsys/Kconfig
@@ -53,4 +53,14 @@
 source "subsys/zbus/Kconfig"
 # zephyr-keep-sorted-stop
 
+config MODULES
+	bool "Make tristate Kconfig options and an 'm' selection available"
+	help
+	  Zephyr supports dynamically loadable code, e.g. using llext. Code,
+	  that can either be built as a part of the system image or as a
+	  loadable extension, can use tristate Kconfig options. For this to work
+	  the CONFIG_MODULES option must be enabled by the project. Enabling
+	  this option alone doesn't change the build on its own, it only allows
+	  using 'm' for tristate Kconfig options.
+
 endmenu