tools: build mkeficapsule with tools-only_defconfig

Add CONFIG_TOOLS_MKEFICAPSULE. Then we want to always build mkeficapsule
if tools-only_defconfig is used.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/Kconfig b/tools/Kconfig
index 91ce8ae..117c921 100644
--- a/tools/Kconfig
+++ b/tools/Kconfig
@@ -90,4 +90,12 @@
 	help
 	  Enable SHA512 support in the tools builds
 
+config TOOLS_MKEFICAPSULE
+	bool "Build efimkcapsule command"
+	default y if EFI_CAPSULE_ON_DISK
+	help
+	  This command allows users to create a UEFI capsule file and,
+	  optionally sign that file. If you want to enable UEFI capsule
+	  update feature on your target, you certainly need this.
+
 endmenu