efi_loader: implement GetNextVariableName()

The current GetNextVariableName() is a placeholder.
With this patch, it works well as expected.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
rebased on efi-next
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>