efi_loader: variable: support variable authentication

With this commit, EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS
is supported for authenticated variables and the system secure state
will transfer between setup mode and user mode as UEFI specification
section 32.3 describes.

Internally, authentication data is stored as part of authenticated
variable's value. It is nothing but a pkcs7 message (but we need some
wrapper, see efi_variable_parse_signature()) and will be validated by
efi_variable_authenticate(), hence efi_signature_verify_with_db().

Associated time value will be encoded in "{...,time=...}" along with
other UEFI variable's attributes.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>