src/lib: add IS_ENABLED() around Kconfig symbol references

Some of these can be changed from #if to if(), but that will happen
in a follow-on commmit.

Change-Id: Idcea3f8b1a4246cb6b29999a84a191a3133e5c78
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20341
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/cbfs.c

@@ -316,7 +316,7 @@ const struct cbfs_locator __attribute__((weak)) cbfs_master_header_locator = {
 extern const struct cbfs_locator vboot_locator;
 
 static const struct cbfs_locator *locators[] = {
-#if CONFIG_VBOOT
+#if IS_ENABLED(CONFIG_VBOOT)
 	&vboot_locator,
 #endif
 	&cbfs_master_header_locator,