efivars: Add compatibility code for compat tasks

It seems people are using 32-bit efibootmgr on top of 64-bit kernels,
which will currently fail horribly when using the efivars interface,
which is the traditional efibootmgr backend (the other being efivarfs).

Since there is no versioning info in the data structure, figure out when
we need to munge the structure data via judicious use of
is_compat_task().

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>