cmd: efidebug: Add support for querying UEFI variable storage

With the previous patches that use OP-TEE and StandAloneMM for UEFI
variable storage we've added functionality for efi_query_variable_info.
So let's add the relevant command to efidebug and retrieve information
about the container used to store UEFI variables

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>