linux: run depmod only if modules are enabled

If the kernel configuration CONFIG_MODULES disabled, there's no
/lib/modules/<version> directory, causing this:

depmod: ERROR: could not open directory [...]/target/lib/modules/4.19.8: No such file or directory
depmod: FATAL: could not search modules: No such file or directory
Makefile:745: recipe for target 'target-finalize' failed

Fixes:

https://gitlab.com/buildroot.org/buildroot/-/jobs/483712831
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712817
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712814
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712796
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712729
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712727
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712698
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712691
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712660
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712573
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712574

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

linux/linux.mk

@@ -539,7 +539,9 @@ endef
 # Run depmod in a target-finalize hook, to encompass modules installed by
 # packages.
 define LINUX_RUN_DEPMOD
-	$(HOST_DIR)/sbin/depmod -a -b $(TARGET_DIR) $(LINUX_VERSION_PROBED)
+	if grep -q "CONFIG_MODULES=y" $(LINUX_DIR)/.config; then \
+		$(HOST_DIR)/sbin/depmod -a -b $(TARGET_DIR) $(LINUX_VERSION_PROBED); \
+	fi
 endef
 LINUX_TARGET_FINALIZE_HOOKS += LINUX_RUN_DEPMOD