Disable unused mkeficapsule

`CONFIG_TOOLS_MKEFICAPSULE` requires gnutls to build. Since we don't actually
expose the tool, we can just disable it.

diff -ur a/configs/tools-only_defconfig b/configs/tools-only_defconfig
--- a/configs/tools-only_defconfig	2023-01-09 09:07:33.000000000 -0700
+++ b/configs/tools-only_defconfig	2023-08-11 14:01:50.151294889 -0600
@@ -33,4 +33,4 @@
 # CONFIG_VIRTIO_SANDBOX is not set
 # CONFIG_GENERATE_ACPI_TABLE is not set
 # CONFIG_EFI_LOADER is not set
-CONFIG_TOOLS_MKEFICAPSULE=y
+# CONFIG_TOOLS_MKEFICAPSULE is not set