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/configs/tools-only_defconfig b/configs/tools-only_defconfig
index 054e008..64eb766 100644
--- a/configs/tools-only_defconfig
+++ b/configs/tools-only_defconfig
@@ -35,3 +35,4 @@
 # CONFIG_VIRTIO_SANDBOX is not set
 # CONFIG_GENERATE_ACPI_TABLE is not set
 # CONFIG_EFI_LOADER is not set
+CONFIG_TOOLS_MKEFICAPSULE=y