configs: enable setvariable at runtime on sandbox

We currently don't have any boards enabling CONFIG_EFI_RT_VOLATILE_STORE.
We do have EFI selftests testing the feature though, so enable it in
all the sandbox platforms and test the functionality properly

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/configs/sandbox64_defconfig b/configs/sandbox64_defconfig
index 2bd4eea..dd0582d 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -267,6 +267,7 @@
 CONFIG_TPM=y
 CONFIG_ERRNO_STR=y
 CONFIG_GETOPT=y
+CONFIG_EFI_RT_VOLATILE_STORE=y
 CONFIG_EFI_SECURE_BOOT=y
 CONFIG_TEST_FDTDEC=y
 CONFIG_UNIT_TEST=y