Allow reading the UEFI variable size

When loading bigger variables form UEFI it is necessary to know their
size beforehand, so that an appropriate amount of memory can be
allocated. The easiest way to do this is to try to read the variable
with buffer size equal 0, expecting EFI_BUFFER_TOO_SMALL error to be
returned. Allow such possible approach in efi_getenv routine.

Extracted from a bigger patch as suggested by imp.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield